perf intel pt decoder: clang has no -Wno-override-init

So set it only for other compilers, allowing us to overcome yet another
build failure due to an inexistent clang -W option:

error: unknown warning option '-Wno-override-init'; did you mean '-Wno-override-module'? [-Werror,-Wunknown-warning-option]

Cc: Adrian Hunter <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Wang Nan <[e-mail]>
Link: http://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/intel-pt-decoder/Build

···
 	$(call rule_mkdir)
 	$(call if_changed_dep,cc_o_c)
 
-CFLAGS_intel-pt-insn-decoder.o += -I$(OUTPUT)util/intel-pt-decoder -Wno-override-init
+CFLAGS_intel-pt-insn-decoder.o += -I$(OUTPUT)util/intel-pt-decoder
+
+ifneq ($(CC), clang)
+  CFLAGS_intel-pt-insn-decoder.o += -Wno-override-init
+endif