Properly create the directory before creating it so that parallel
install work fine.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/Makefile
===================================================================
--- a/Makefile
+++ b/Makefile
@@ -22,6 +22,7 @@
 	install -D --mode=644 40-usb_modeswitch.rules $(RULESDIR)/40-usb_modeswitch.rules
 
 db-install:
+	install -d $(PREFIX)/share/usb_modeswitch
 	install --mode=644 -t $(PREFIX)/share/usb_modeswitch ./usb_modeswitch.d/*
 
 db-install-packed: