cmake: Fix a typo in FindLibUSB.cmake

This typo made it unclear why libusb is not found.
Reported in #459, #512, #458, #495.

Author: xlz

cmake_modules/FindLibUSB.cmake

@@ -16,7 +16,7 @@ IF(PKG_CONFIG_FOUND)
     SET(MODULE "libusb-1.0>=1.0.20")
   ENDIF()
   IF(LibUSB_FIND_REQUIRED)
-    SET(LisUSB_REQUIRED "REQUIRED")
+    SET(LibUSB_REQUIRED "REQUIRED")
   ENDIF()
   PKG_CHECK_MODULES(LibUSB ${LibUSB_REQUIRED} ${MODULE})