MINOR: build: Disable -Wstringop-overflow.

Disable -Wstringop-overflow, as it gives annoying false positives
with gcc 8.
diff --git a/Makefile b/Makefile
index 424d98c..821d314 100644
--- a/Makefile
+++ b/Makefile
@@ -167,6 +167,7 @@
 SPEC_CFLAGS += $(call cc-nowarn,clobbered)
 SPEC_CFLAGS += $(call cc-nowarn,missing-field-initializers)
 SPEC_CFLAGS += $(call cc-nowarn,implicit-fallthrough)
+SPEC_CFLAGS += $(call cc-nowarn,stringop-overflow)
 SPEC_CFLAGS += $(call cc-opt,-Wtype-limits)
 SPEC_CFLAGS += $(call cc-opt,-Wshift-negative-value)
 SPEC_CFLAGS += $(call cc-opt,-Wshift-overflow=2)