Makefile: Set -Wno-unused-but-set-variable globally

Apparently nobody seems to have tested clang 13 before release, as the whole
coding paradigm of calling a function by assigning its return value to a
variable, which is used all throughout the Linux kernel, becomes broken,
as clang seems utterly drunk when it comes to the actual judgement whether
the variable is really used or not..

Makefile

@@ -482,7 +482,7 @@ KBUILD_AFLAGS   := -D__ASSEMBLY__ -fno-PIE
 KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
 		   -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE \
 		   -Werror=implicit-function-declaration -Werror=implicit-int \
-		   -Wno-format-security \
+		   -Wno-format-security -Wno-unused-but-set-variable \
 		   -std=gnu89
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_AFLAGS_KERNEL :=