gettid.2: Document header file and feature test macro requirements for gettid()

The gettid() wrapper was added glibc 2.30, and is declared by
<unistd.h> if _GNU_SOURCE is defined.

Reported-by: Joseph C. Sible <josephcsible@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/gettid.2

@@ -28,6 +28,8 @@
 gettid \- get thread identification
 .SH SYNOPSIS
 .nf
+.B #define _GNU_SOURCE
+.B #include <unistd.h>
 .B #include <sys/types.h>
 .PP
 .B pid_t gettid(void);