Create $(STAGING_DIR)/usr/bin unconditionnally

This fixes the build of libcap, with external toolchain, when it is
the first package being built.

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

Makefile

@@ -371,6 +371,7 @@ ifneq ($(BR2_TOOLCHAIN_EXTERNAL),y)
 endif
 endif
 	@mkdir -p $(STAGING_DIR)/usr/include
+	@mkdir -p $(STAGING_DIR)/usr/bin
 
 $(BUILD_DIR)/.root:
 	mkdir -p $(TARGET_DIR)