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 b21ddbfa18cd1e437e31dc0fa29408721a186c2a)