compiler/glsl: fix include for Android build
Sources outside of src/util path should include "util/string_buffer.h"
Fixes the following building error in Android:
external/mesa/src/compiler/glsl/ast_type.cpp:25:10: fatal error: 'string_buffer.h' file not found ^~~~~~~~~~~~~~~~~ 1 error generated.
Fixes: eeec9d56 ("compiler/glsl: clean up output")