man4: Only link mgb.4 to if_mgb.4 where mgb.4 is installed

mgb.4 is only installed on amd64 and i386, move the MLINKS entry into
the amd64/i386 block next to _mgb.4.

Fixes:		05c49e2aa6 ("man: Link mgb.4 to if_mgb.4")
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation

share/man/man4/Makefile

@@ -797,7 +797,6 @@ 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
@@ -953,6 +952,7 @@ _wpi.4=		wpi.4
 _xen.4=		xen.4
 _xnb.4=		xnb.4
 
+MLINKS+=mgb.4 if_mgb.4
 .endif
 
 .if ${MACHINE_CPUARCH} == "amd64"