gallium/tests: fix build with clang compiler
gallium/tests: fix build with clang compiler

Nested functions are supported as an extension in GNU C, but Clang
don't support them.

This fixes compilation errors when (manually) building compute.c,
or by setting --enable-gallium-tests to the configure script.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75165
Signed-off-by: Samuel Pitoiset <506e97eed81d9e59d22f4c73fb51123bb62b4a30@gmail.com>
Reviewed-by: Edward O'Callaghan <117173807d5a49976a891944633465c0daea7f98@alterapraxis.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler