make: Disable -Wpoison-system-directories

This compiler warning is used for cross-compilation only,
which is not used for hyperkit. Explicitly disable it.

Signed-off-by: Rolf Neugebauer <rn@rneugeba.io>

Author: rn

config.mk

@@ -45,7 +45,8 @@ CFLAGS_WARN := \
   -Wno-reserved-id-macro \
   -Wno-unknown-warning-option \
   -Wno-unused-macros \
-  -Wno-switch-enum
+  -Wno-switch-enum \
+  -Wno-poison-system-directories
 
 CFLAGS_DIAG := \
   -fmessage-length=152 \