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 -Naur linux-5.9.orig/tools/perf/Makefile.config linux-5.9/tools/perf/Makefile.config
--- linux-5.9.orig/tools/perf/Makefile.config	2020-10-17 11:55:52.631117295 +0300
+++ linux-5.9/tools/perf/Makefile.config	2020-10-17 11:55:54.391117325 +0300
@@ -269,6 +269,7 @@
 CORE_CFLAGS += -Wall
 CORE_CFLAGS += -Wextra
 CORE_CFLAGS += -std=gnu99
+CORE_CFLAGS += -Wno-format-truncation -Wno-format-overflow
 
 CXXFLAGS += -std=gnu++11 -fno-exceptions -fno-rtti
 CXXFLAGS += -Wall