Fixed include path when using internal zlib.

CMakeLists.txt

@@ -293,7 +293,7 @@ IF(PHYSFS_NEED_ZLIB)
     ENDIF(ZLIB_FOUND)
 
     IF(PHYSFS_INTERNAL_ZLIB)
-        INCLUDE_DIRECTORIES(zlib123)
+        INCLUDE_DIRECTORIES(./src/zlib123)
         ADD_DEFINITIONS(-DZ_PREFIX=1)
         SET(PHYSFS_SRCS ${PHYSFS_SRCS} ${ZLIB_SRCS})
     ELSE(PHYSFS_INTERNAL_ZLIB)