st/mesa: Silence GCC unused-variable warning.

st/mesa: Silence GCC unused-variable warning.

Silence a release build warning.

st_glsl_to_tgsi.cpp: In function 'pipe_error st_translate_program(gl_context*, uint, ureg_program*, glsl_to_tgsi_visitor*, const gl_program*, GLuint, const GLuint*, const GLuint*, const ubyte*, const ubyte*, const GLuint*, const GLuint*, GLuint, const GLuint*, const GLuint*, const ubyte*, const ubyte*, boolean, boolean)':
st_glsl_to_tgsi.cpp:5461:36: warning: unused variable 'pscreen' [-Wunused-variable]
                struct pipe_screen *pscreen = st->pipe->screen;
                                    ^

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>