[MIPS] Add empty argument parenthesis to GCC_IMM_ASM

This is to clarify that GCC_IMM_ASM does not take an argument as the
context of the macro's invocation seems to imply.

As suggested by Maciej W. Rozycki (macro@linux-mips.org).

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

include/asm-mips/compiler.h

@@ -9,10 +9,10 @@
 #define _ASM_COMPILER_H
 
 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
-#define GCC_IMM_ASM "n"
+#define GCC_IMM_ASM() "n"
 #define GCC_REG_ACCUM "$0"
 #else
-#define GCC_IMM_ASM "rn"
+#define GCC_IMM_ASM() "rn"
 #define GCC_REG_ACCUM "accum"
 #endif