kbuild: drop -Wundef from HOSTCFLAGS for now

-Wundef caused warnings in the bison generated code in kconfig.
Updating to a newer bison (1.875d) did not fix it. The alternatives
was to correct the autogenerated code or drop -Wundef.
For now -Wundef is dropped from HOSTCFLAGS.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
---

Makefile

@@ -203,7 +203,7 @@ CONFIG_SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \
 
 HOSTCC  	= gcc
 HOSTCXX  	= g++
-HOSTCFLAGS	= -Wall -Wundef -Wstrict-prototypes -O2 -fomit-frame-pointer
+HOSTCFLAGS	= -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer
 HOSTCXXFLAGS	= -O2
 
 # 	Decide whether to build built-in, modular, or both.