include what you use: errno.h in exec_process.cc

Add missing errno.h inclusion - this is less likely to
break uncommon toolchains (eg uClibc).  And while we're
at it, merge posix includes into else case for an
#ifdef OS_WIN block to match the code that uses these
includes.

R=brettw@chromium.org,tfarina@chromium.org

Review URL: https://codereview.chromium.org/1421293008

Cr-Commit-Position: refs/heads/master@{#358944}
1 file changed