libportable: Add dependency on libdl.
The patch at [1] was reverted because ndk/sources/android/cpu-features
is now part of the platform build (as a dependency of libportable), and
now requires dlopen()/dlsym()/dlclose().
Because the platform build system doesn't support LOCAL_EXPORT_LDLIBS,
add the dependency to libportable's own module definition instead.
After this patch is submitted, it's possible to re-apply the original
patch and get a working build.
[1] https://android-review.googlesource.com/#/c/91380/
BUG=13679666
Change-Id: I54b4376a562242d21631eaf13090650decfe791d
1 file changed