[DFS] fix the compiling issue under Keil MDK.

components/dfs/include/dfs_posix.h

@@ -30,6 +30,10 @@
 #include <dfs_file.h>
 #include <dfs_def.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #ifndef RT_USING_NEWLIB
 #define O_RDONLY    DFS_O_RDONLY
 #define O_WRONLY    DFS_O_WRONLY
@@ -79,6 +83,7 @@
 
 #if defined(__CC_ARM)
 #include <stdio.h>
+#include <stdlib.h>
 #elif defined(_MSC_VER)
 #include <stdio.h>
 #else
@@ -109,6 +114,8 @@ int closedir(DIR* d);
 #include <sys/stat.h>
 #endif
 
+struct stat;
+
 /* file api*/
 int open(const char *file, int flags, int mode);
 int close(int d);
@@ -126,5 +133,8 @@ int rmdir(const char *path);
 int chdir(const char *path);
 char *getcwd(char *buf, size_t size);
 
+#ifdef __cplusplus
+}
 #endif
 
+#endif