tools/testing/selftests/powerpc: remove redundant CFLAGS in Makefile: "-Wall -O2 -Wall" -> "-O2 -Wall"

There are repeated compiler flags "-Wall" in the Makefile.

The redundant compiler flag appears to have been there from the
 introduction of the Makefile.

Signed-off-by: Chijun Sima <simachijun@qq.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

tools/testing/selftests/powerpc/Makefile

@@ -8,7 +8,7 @@ ifeq ($(ARCH),powerpc)
 
 GIT_VERSION = $(shell git describe --always --long --dirty || echo "unknown")
 
-CFLAGS := -std=gnu99 -Wall -O2 -Wall -Werror -DGIT_VERSION='"$(GIT_VERSION)"' -I$(CURDIR)/include $(CFLAGS)
+CFLAGS := -std=gnu99 -O2 -Wall -Werror -DGIT_VERSION='"$(GIT_VERSION)"' -I$(CURDIR)/include $(CFLAGS)
 
 export CFLAGS