libusb: disable parallel building

The libusb package is not parallel build save, a make -j16 reliably breaks it.
Forcibly disable parallel building.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/libs/libusb/Makefile

@@ -16,7 +16,7 @@ PKG_SOURCE_URL:=@SF/$(PKG_NAME)
 PKG_MD5SUM:=1d4eb194eaaa2bcfbba28102768c7dbf
 
 PKG_INSTALL:=1
-PKG_BUILD_PARALLEL:=1
+PKG_BUILD_PARALLEL:=0
 PKG_LICENSE:=LGPL-2.1
 
 PKG_MAINTAINER := Felix Fietkau <nbd@openwrt.org>