Fix cmake build failure under linux

The LIBUSB_FOUND variable was used before the libusb detection was performed.
Fixes issue 240.

CMakeLists.txt

@@ -111,6 +111,8 @@ IF (WIN32)
   ENDIF(PCRE_INCLUDE_DIRS)
 ENDIF(WIN32)
 
+INCLUDE(LibnfcDrivers)
+
 IF(PCSC_INCLUDE_DIRS)
   INCLUDE_DIRECTORIES(${PCSC_INCLUDE_DIRS})
   LINK_DIRECTORIES(${PCSC_LIBRARY_DIRS})

libnfc/CMakeLists.txt

@@ -19,8 +19,6 @@ ELSE(LIBUSB_FOUND)
 ENDIF(LIBUSB_FOUND)
 INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR}/buses)
 
-INCLUDE(LibnfcDrivers)
-
 IF(WIN32)
   # Windows now requires regex, so we utilize PCRE
   # since Windows doesn't get the benefit of finding in CMake