fix devel/libatomic_ops on sparc

[Tobias Ulmer <tobiasu%tmux.org@localhost>]

MACHINE_PLATFORM isn't set without including bsd.prefs.mk

Tested with www/w3m

Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/devel/libatomic_ops/Makefile,v
retrieving revision 1.20
diff -u -p -r1.20 Makefile
--- Makefile	2 Mar 2019 14:51:53 -0000	1.20
+++ Makefile	24 Apr 2020 10:13:26 -0000
@@ -19,6 +19,8 @@ TEST_TARGET=		check
 
 INSTALL_MAKE_FLAGS=	pkgdatadir=${PREFIX}/share/doc/libatomic_ops
 
+.include "../../mk/bsd.prefs.mk"
+
 .if !empty(MACHINE_PLATFORM:MNetBSD-*-sparc)
 CPPFLAGS+=	-DAO_NO_SPARC_V9=1
 .endif