tool/ports/current: use silent option for GNU Make 4.4

If this file is called from a Makefile via

! $(shell <path>/tool/ports/current qt5-host)

'shell' will return

"Entering directory '' <path-to-libc> Leaving directory '' Stop.",

and thus, producing multiple targets in the 'tool_chain_qt5' case.

issue #4725

tool/ports/current

@@ -1,4 +1,4 @@
-#!/usr/bin/make -f
+#!/usr/bin/make -sf
 
 #
 # \brief  Tool for printing current contrib directory of port