man: Link mgb.4 to if_mgb.4

For consistency, create a symbolic link from mgb.4 to also
if_mgb.4

Reviewed by:		#manpages, ziaee, emaste
Differential Revision:	https://reviews.freebsd.org/D60551
MFC after:		3 days

share/man/man4/Makefile

@@ -797,6 +797,7 @@ MLINKS+=me.4 if_me.4
 MLINKS+=mem.4 kmem.4
 MLINKS+=mfi.4 mfi_linux.4 \
 	mfi.4 mfip.4
+MLINKS+=mgb.4 if_mgb.4
 MLINKS+=mlx5en.4 mce.4 \
 	mlx5en.4 if_mce.4
 MLINKS+=mos.4 if_mos.4