Export _mmap and __sys_mmap from libc.so

Unlike the other syscalls these two symbols were missing from the
version script. I noticed this while looking into the compiler-rt
runtime libraries for CHERI.

Reviewed by:	brooks
Obtained from:	https://github.com/CTSRD-CHERI/cheribsd/pull/1063
MFC after:	3 days

lib/libc/sys/Symbol.map

@@ -765,6 +765,8 @@ FBSDprivate_1.0 {
 	__sys_mlock;
 	_mlockall;
 	__sys_mlockall;
+	_mmap;
+	__sys_mmap;
 	_modfind;
 	__sys_modfind;
 	_modfnext;