Document that fnctl.h is required for mq_open(2)

[felix.the.red_gmail.com]

Summary

mqueue_open.2: Add an include line to show that fnctl.h is required.

From the original PR:
Users of the standard library mq_open() function will likely need to make
use of one or more of the O_* macros mentioned in the relevant man page,
e.g. O_CREAT.

Unfortunately, the man page for mq_open() only suggest inclusion of the
<mqueue.h> system-supplied header file.
It should also suggest inclusion of <fcntl.h> which is needed in order to
also drag in the various O_* macro definitions.

PR:243099

Test Plan

Try to compile test C file. It does not compile by default.
If the #include <fnctl.h> line is uncommented, test C file{F121620696} compiles.