go/go-package.mk: Do not print lines in print-PLIST unconditionally

Previously all lines (containing or not /${GO_PLATFORM}/) were printed
unconditionally and could interfere with other PRINT_PLIST_AWKs used.
Avoid that by limiting the pattern.

Index: go-package.mk
===================================================================
RCS file: /cvsroot/pkgsrc/lang/go/go-package.mk,v
retrieving revision 1.11
diff -u -p -r1.11 go-package.mk
--- go-package.mk	27 Jan 2018 15:49:30 -0000	1.11
+++ go-package.mk	5 Jun 2018 15:29:56 -0000
@@ -66,7 +66,7 @@ USE_TOOLS+=		pax
 GO_PLATFORM=		${LOWER_OPSYS}_${GOARCH}
 GOTOOLDIR=		${PREFIX}/go/pkg/tool/${GO_PLATFORM}
 
-PRINT_PLIST_AWK+=	{ gsub(/${GO_PLATFORM}/, \
+PRINT_PLIST_AWK+=	/${GO_PLATFORM}/ { gsub(/${GO_PLATFORM}/, \
 			"$${GO_PLATFORM}"); \
 			print; next; }