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 <[email protected]>

toolchain/Config.in

@@ -169,6 +169,7 @@ menuconfig EXTERNAL_TOOLCHAIN
 		string
 		prompt "Toolchain include path"  if DEVEL
 		depends on EXTERNAL_TOOLCHAIN && !NATIVE_TOOLCHAIN
+		default "./usr/include ./include/fortify ./include" if EXTERNAL_TOOLCHAIN_LIBC_USE_MUSL
 		default "./usr/include ./include"
 		help
 		  Specify additional directories searched for header files (override