kpatch-build: set C language standard to gnu11

Add -std=gnu11 to CFLAGS for kpatch-build tooling.  This aligns with the
kernel build and avoids confusion when older tooling may default to
earlier versions.

Closes: dynup#1416 ("C99 code vs. gcc defaults?")
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>

kpatch-build/Makefile

@@ -1,6 +1,6 @@
 include ../Makefile.inc
 
-CFLAGS += -MMD -MP -I../kmod/patch -Iinsn -Wall -Wsign-compare \
+CFLAGS += -std=gnu11 -MMD -MP -I../kmod/patch -Iinsn -Wall -Wsign-compare \
 	  -Wconversion -Wno-sign-conversion -g -Werror
 LDLIBS = -lelf