Disable _FORTIFY_SOURCE to fix Ubuntu build with -Werror

This eliminates the results unused warnings.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/Makefile b/Makefile
index dabf322..a19950c 100644
--- a/Makefile
+++ b/Makefile
@@ -22,6 +22,7 @@
 
 CPPFLAGS += -I. -I$(SRC_PATH) -MMD -MP -MT $@
 CPPFLAGS += -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
+CPPFLAGS += -U_FORTIFY_SOURCE
 LIBS=
 ifdef CONFIG_STATIC
 LDFLAGS += -static