Fix compilation under clang-19 aarch64

[andrjohns]

The development version is failing to compile under the rtools45 pre-release for Windows Arm because one of the methods in the tbb-compat stub is missing an attribute from one the declarations:

Installing package into 'C:/Users/andrew/AppData/Local/R/aarch64-library/4.6'
(as 'lib' is unspecified)
* installing *source* package 'RcppParallel' ...
** this is package 'RcppParallel' version '5.1.10.9000'
** using staged installation
** preparing to configure package 'RcppParallel' ...
*** configured file: 'R/tbb-autodetected.R.in' => 'R/tbb-autodetected.R'
*** configured file: 'src/Makevars.in' => 'src/Makevars'
** finished configure for package 'RcppParallel'
** libs
using C++ compiler: 'clang version 19.1.7'
installing via 'install.libs.R' to C:/Users/andrew/AppData/Local/R/aarch64-library/4.6/00LOCK-RcppParallel/00new/RcppParallel
** creating tbb stub library
using C++ compiler: 'clang version 19.1.7'
clang++ -std=gnu++17  -I"C:/PROGRA~1/R-AARC~1/R-devel/include" -DNDEBUG -I../inst/include -I"C:/rtools45-aarch64/aarch64-w64-mingw32.static.posix/include"    -I"/aarch64-w64-mingw32.static.posix/include"   -DRCPP_PARALLEL_USE_TBB=1   -O2 -Wall      -c tbb-compat/tbb-compat.cpp -o tbb-compat/tbb-compat.o
tbb-compat/tbb-compat.cpp:137:54: error: redeclaration of 'tbb::internal::task_scheduler_observer_v3::observe' cannot add 'dllexport' attribute
  137 | void __TBB_EXPORTED_FUNC task_scheduler_observer_v3::observe( bool enable ) {
      |                                                      ^
tbb-compat/tbb-compat.cpp:43:31: note: previous declaration is here
   43 |    void __TBB_EXPORTED_METHOD observe( bool state=true );
      |                               ^
1 error generated.
make: *** [C:/PROGRA~1/R-AARC~1/R-devel/etc/Makeconf:296: tbb-compat/tbb-compat.o] Error 1
Error in .install.libs(tbbLib) : error building tbb stub library
* removing 'C:/Users/andrew/AppData/Local/R/aarch64-library/4.6/RcppParallel'
* restoring previous 'C:/Users/andrew/AppData/Local/R/aarch64-library/4.6/RcppParallel'
Warning message:
In install.packages(getwd(), type = "source", repos = NULL) :

This errors since clang has different handling of attributes in this case, and attribute needs to be present in both declarations

[kevinushey]

Thanks for the fix!