osmesa: Add OSMesaColorClamp and OSMesaGetProcAddress to symbol defs.

Without this patch, the two symbols get an underscore prepended
and an "@4" appended when compiling with VC8.

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

(cherry picked from commit e65029e9b32ddabea0ec583c04484345b40f1557)
1 file changed