Android: Fix support for CMAKE_SYSROOT without CMAKE_SYSROOT_COMPILE
If CMAKE_SYSROOT
is set by the user (e.g. via the toolchain file) then
we do not set CMAKE_SYSROOT_COMPILE
to our default. Fix our
references to the sysroot's /usr/include
directory to use
CMAKE_SYSROOT_COMPILE
only if it is set and else CMAKE_SYSROOT
.
Fixes: #17096 (closed)