Fix cmake warning

CMake Warning (dev) at utils/CMakeLists.txt:50 (ADD_EXECUTABLE):
  Policy CMP0115 is not set: Source file extensions must be explicit.  Run
  "cmake --help-policy CMP0115" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  File:

    /home/rousseau/Documents/github/libnfc/utils/jewel.c
This warning is for project developers.  Use -Wno-dev to suppress it.

utils/CMakeLists.txt

@@ -30,16 +30,16 @@ FOREACH(source ${UTILS-SOURCES})
   ENDIF(WIN32)
 
   IF(${source} MATCHES "nfc-jewel")
-    LIST(APPEND TARGETS jewel)
+    LIST(APPEND TARGETS jewel.c)
   ENDIF(${source} MATCHES "nfc-jewel")
 
   IF((${source} MATCHES "nfc-mfultralight") OR (${source} MATCHES "nfc-mfclassic"))
-    LIST(APPEND TARGETS mifare)
+    LIST(APPEND TARGETS mifare.c)
   ENDIF((${source} MATCHES "nfc-mfultralight") OR (${source} MATCHES "nfc-mfclassic"))
 
   IF(WIN32)
     IF(${source} MATCHES "nfc-scan-device")
-      LIST(APPEND TARGETS ../contrib/win32/stdlib)
+      LIST(APPEND TARGETS ../contrib/win32/stdlib.c)
       INCLUDE_DIRECTORIES(${CMAKE_CURRENT_SOURCE_DIR}/../contrib/win32)
     ENDIF(${source} MATCHES "nfc-scan-device")
     IF(${source} MATCHES "nfc-read-forum-tag3")