Revert "tools flex: disable parallel build (#3169)" (#3350)

This reverts commit b4807e75.

The race condition has been fixed in OpenWrt.

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 177136b78c77c31c56db2fc993d1273d3dd81562..cb4f95e8d0c45a9c328c7ce48235a0ba80c02c6e 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
-