glsl: Remove unused include from builtin_variables.cpp

Found with IWYU. Compile-tested on my Ivy-bridge system.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Thomas Helland <thomashelland90@gmail.com>

src/glsl/builtin_variables.cpp

@@ -26,7 +26,6 @@
 #include "glsl_symbol_table.h"
 #include "main/core.h"
 #include "main/uniforms.h"
-#include "program/prog_parameter.h"
 #include "program/prog_statevars.h"
 #include "program/prog_instruction.h"