postfix: fix musl compatibility

postfix: fix musl compatibility

The current package patches `makedefs` to probe for the existance
of the `nsl` and `resolv` libraries but uses the wrong variables
for the prefix, leading to `makedefs` probing the host directories,
not the staging dir ones.

Replace `$PKG_BUILD_DIR` with `$STAGING_DIR` to perform the library
tests in the correct directory.

Fixes the following error spotted by doing test builds against musl
which does not provide a `libnsl`:

    .../mips-openwrt-linux-musl/bin/ld: cannot find -lnsl
    collect2: error: ld returned 1 exit status
    make: *** [master] Error 1

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>