Turn off errors for -Wmaybe-uninitialized in GCC 6+.

Recent changes to <sys/tree.h> trigger this warning and seem like a
false positive.

Differential Revision:	https://reviews.freebsd.org/D25726

share/mk/bsd.sys.mk

@@ -150,7 +150,8 @@ CWARNFLAGS+=	-Wno-error=address			\
 
 # GCC 6.1.0
 .if ${COMPILER_VERSION} >= 60100
-CWARNFLAGS+=	-Wno-error=nonnull-compare		\
+CWARNFLAGS+=	-Wno-error=maybe-uninitialized		\
+		-Wno-error=nonnull-compare		\
 		-Wno-error=shift-negative-value		\
 		-Wno-error=tautological-compare		\
 		-Wno-error=unused-const-variable