Assume bionic has up-to-date linux/capability.h

The fallback android_filesystem_capability.h doesn't play nicely with
other kernel headers, since it #undef's __user. If we're building with
bionic (either for device or host), we use the same kernel headers, so
just use those.

Bug: 38056396
Test: build with Host_bionic:true
Change-Id: Idc61b6d96d86891164abe71604924638d67aefe2