perf: Silence two kinds of format warnings from GCC 8

perf may be fixed properly in the newer kernel versions but for now, it is
easier to just silence -Werror=format-overflow and -Werror=format-truncation.
---

diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
index 3ec1969b6d3e..3d924b43db4f 100644
--- a/tools/perf/Makefile.config
+++ b/tools/perf/Makefile.config
@@ -203,6 +203,7 @@ CFLAGS += -funwind-tables
 CFLAGS += -Wall
 CFLAGS += -Wextra
 CFLAGS += -std=gnu99
+CFLAGS += -Wno-format-truncation -Wno-format-overflow
 
 CXXFLAGS += -std=gnu++11 -fno-exceptions -fno-rtti
 CXXFLAGS += -Wall
--