glsl: fix compiler warnings in link_uniforms.cpp

glsl: fix compiler warnings in link_uniforms.cpp

With a non-debug build, gcc has two complaints:
1. 'found' var not used.  Silence with '(void) found;'
2. 'id' not initialized.  It's assigned by the UniformHash->get()
   call, actually.  But init it to zero to silence gcc.

Reviewed-by: Matt Turner <mattst88@gmail.com>