arm/gn: disable uninitialized variable warning

An uninitialized variable warning is occurring when compiling gn.
However, since Wall is being used in gn, this is being logged as an
error.  Pass the no-error flag for this case to workaround this issue.

Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/recipes-devtools/gn/gn_git.bb

@@ -43,3 +43,4 @@ do_install() {
 BBCLASSEXTEND = "native"
 
 COMPATIBLE_HOST = "^(?!riscv32).*"
+CFLAGS += "-Wno-error=maybe-uninitialized"