toolchain: Include ./include/fortify for external musl toolchain

toolchain: Include ./include/fortify for external musl toolchain

When building with an external toolcahin with musl also include
./include/fortify by default. This is also done when we build with the
internal toolchain using musl libc.

Without this extra include the fortify source feature is not working
when using an external musl toolchain. All binaries were compiled
without fortify source when an external musl toolchain was used. All
binaries release done by the OpenWrt project use the internal toolcahin
where fortify source is working.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 3053045093a5c89dad970b7db3ed9f2a5be993f6)
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>