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: eeec9d56ad ("compiler/glsl: clean up output")
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10156>

src/compiler/glsl/ast_type.cpp

@@ -22,7 +22,7 @@
  */
 
 #include "ast.h"
-#include "string_buffer.h"
+#include "util/string_buffer.h"
 
 void
 ast_type_specifier::print(void) const