#182 uuid.c: add asm/unistd.h include to fix build with musl-1.2.0

[Bernd Kuhls]

Build was broken due to musl commit
https://git.musl-libc.org/cgit/musl/commit/?id=5a105f19b5aae79dd302899e634b6b18b3dcd0d6

To fix also include asm/unistd.h following the configure.ac check for
the __NR_clock_gettime syscall.

Fixes buildroot build error:
http://autobuild.buildroot.net/results/31b/31bf9814a5a45081cf58675a9f7fa7183b4ef0bf/