crossgcc/buildgcc: Add missing "\" at build_NASM

"\" is missing at the end of CC line for build_NASM.

Change-Id: Ic29ee731def31f958f939efe19bdb55b503eb6ba
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/72512
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

util/crossgcc/buildgcc

@@ -898,7 +898,7 @@ build_CMAKE() {
 }
 
 build_NASM() {
-	CC="$(hostcc host)" CXX="$(hostcxx host)" CFLAGS="$HOSTCFLAGS"
+	CC="$(hostcc host)" CXX="$(hostcxx host)" CFLAGS="$HOSTCFLAGS" \
 	../${NASM_DIR}/configure \
 		--prefix="$TARGETDIR" || touch .failed
 	# shellcheck disable=SC2086