uclibc-ng-test: fix build with musl and older kernel headers

uclibc-ng-test: fix build with musl and older kernel headers

musl libc provides its own system call macros, including
preadv2/pwritev2. Having these macros defined is not enough to determine
availability of other related macros in kernel headers.

Add a patch to check these macros directly.

Fixes:
http://autobuild.buildroot.net/results/7e9/7e9945f46ef9e0f6de7e1c85b0ca1104df6c86f5/
http://autobuild.buildroot.net/results/782/782652193bb6770ae68d509b3a7087667622d627/
http://autobuild.buildroot.net/results/582/5827a724c7f24218010f4cdd24924dca457b0361/

Cc: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>