mesa: Wrap extensions.h declarations with extern "C".
mesa: Wrap extensions.h declarations with extern "C".

This should fix the MSVC linker failures that arose with commit
5e2d25894b962aae9158261897e13843377e3b95.

Trivial.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler