mesa: cast the GLenum16 to GLint to avoid compile warning on android

mesa: cast the GLenum16 to GLint to avoid compile warning on android

Cast the enum to GLint to avoid the compile warning:
/src/mesa/main/get.c:3005:19:
warning: comparison of constant -32768 with expression of type
'GLenum16' (aka 'unsigned short') is always false
-Wtautologicalia-constant-out-of-range-compare

Tests: compilation without this warning
Signed-off-by: jenny.q.cao <jenny.q.cao@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>