prevent multiply defined symbols

prevent multiply defined symbols

Without this patch applied gcc@10.1.0 fails to compile with the following
error (note mesa@18.3.6 but the latest release also posseses this problem):

  ld: ../../../../src/gallium/auxiliary/.libs/libgallium.a(u_debug_symbol.o):/tmp/spack/spack-stage/spack-stage-mesa-18.3.6-be7kyg2dyxwktir3zrai27n6a6coadab/spack-src/src/galli um/auxiliary/util/u_debug_symbol.c:273: multiple definition of `symbols_hash'; ../../../../src/gallium/auxiliary/.libs/libgallium.a(u_debug_stack.o):/tmp/spa ck/spack-stage/spack-stage-mesa-18.3.6-be7kyg2dyxwktir3zrai27n6a6coadab/spack-src/src/gallium/auxiliary/util/u_debug_stack.c:49: first defined here
  collect2: error: ld returned 1 exit status
  make[4]: *** [libGL.la] Error 1
  make[4]: Leaving directory `/tmp/spack/spack-stage/spack-stage-mesa-18.3.6-be7kyg2dyxwktir3zrai27n6a6coadab/spack-src/src/gallium/targets/libgl-xlib'
  make[3]: *** [all-recursive] Error 1
  make[3]: Leaving directory `/tmp/spack/spack-stage/spack-stage-mesa-18.3.6-be7kyg2dyxwktir3zrai27n6a6coadab/spack-src/src/gallium'
  make[2]: *** [all-recursive] Error 1
  make[2]: Leaving directory `/tmp/spack/spack-stage/spack-stage-mesa-18.3.6-be7kyg2dyxwktir3zrai27n6a6coadab/spack-src/src'
  make[1]: *** [all] Error 2
  make[1]: Leaving directory `/tmp/spack/spack-stage/spack-stage-mesa-18.3.6-be7kyg2dyxwktir3zrai27n6a6coadab/spack-src/src'
  make: *** [all-recursive] Error 1

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3298
Cc: mesa-stable
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6053>