mesa: Fix const correctness of API implementation functions.

mesa: Fix const correctness of API implementation functions.

This patch changes the use of const in the type signatures of
_mesa_ShaderSource() and _mesa_TransformFeedbackVaryings(), to match
the type signatures in the GL spec.  This avoids warnings when
building the code-generated api_exec.c file.

Note: previously we avoided the build warnings because these functions
were being type-checked against ShaderSourceARB and
TransformFeedbackVaryingsEXT; those functions are semantically
equivalent, but have fewer const qualifiers in their type signatures.

Acked-by: Brian Paul <brianp@vmware.com>