tools flex: disable parallel build (#3169)

Sometimes the build of flex fails with erros like these:

./build-aux/config.sub: line 63: opying: command not found

Disable the parallel build of flex in order to avoid these issues from
occuring.

Signed-off-by: David Bauer <mail@david-bauer.net>

patches/openwrt/0008-tools-flex-disable-parallel-build.patch

+From: David Bauer <mail@david-bauer.net>
+Date: Sun, 21 Jan 2024 02:03:56 +0100
+Subject: tools flex: disable parallel build
+
+Sometimes the build of flex fails with erros like these:
+
+./build-aux/config.sub: line 63: opying: command not found
+
+Disable the parallel build of flex in order to avoid these issues from
+occuring.
+
+Signed-off-by: David Bauer <mail@david-bauer.net>
+
+diff --git a/tools/flex/Makefile b/tools/flex/Makefile
+index 86ba5a4415d3a82aaf5e9897736b465cc7e30c42..f3b6d0b1495b30c278e5bf0d2b322b5ade79ec4f 100644
+--- a/tools/flex/Makefile
++++ b/tools/flex/Makefile
+@@ -15,7 +15,6 @@ PKG_SOURCE_URL:=https://github.com/westes/flex/releases/download/v$(PKG_VERSION)
+ PKG_HASH:=e87aae032bf07c26f85ac0ed3250998c37621d95f8bd748b31f15b33c45ee995
+ 
+ HOST_FIXUP:=autoreconf
+-HOST_BUILD_PARALLEL:=1
+ 
+ include $(INCLUDE_DIR)/host-build.mk
+