compat.h: Add MADV_MERGEABLE and MADV_UNMERGEABLE.

This patch fixes these build errors with glibc 2.11 and older.

syscalls/madvise.c:28: error: ‘MADV_MERGEABLE’ undeclared here (not in a function)
syscalls/madvise.c:28: error: ‘MADV_UNMERGEABLE’ undeclared here (not in a function)

Signed-off-by: Vinson Lee <vlee@twitter.com>