i965: Return the correct value type from brw_compile_gs()

i965: Return the correct value type from brw_compile_gs()

brw_compile_gs() should return a pointer to unsigned, but it is returning the
bool 'false' at some point, hence annoying us with a compiler warning:

In function 'const unsigned int* brw::brw_compile_gs(const brw_compiler*,
   void*, void*, const brw_gs_prog_key*, brw_gs_prog_data*, const nir_shader*,
   gl_shader_program*, int, unsigned int*, char**)':

brw_vec4_gs_visitor.cpp:776:14: warning: converting 'false' to pointer type
                                'const unsigned int*' [-Wconversion-null]
                                return false;
                                       ^
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>