kservice: export vsnprintf as rt_vsnprintf

vsnprintf is a common string function that could be used in many places.
Using both vsnprintf in libc and vsnprintf in the RTT could make a
bigger image. Moreover, if newlib is not enabled when compiling with
GCC, referencing vsnprintf will lead to link error:

    .../arm-none-eabi/lib/armv7-ar/thumb/softfp/libc.a(lib_a-sbrkr.o):
    In function `_sbrk_r':
    sbrkr.c:(.text._sbrk_r+0xc): undefined reference to `_sbrk'
    collect2: error: ld returned 1 exit status

Using rt_vsnprintf could avoid such problem.

include/rtthread.h

@@ -469,6 +469,7 @@ int rt_system_module_init(void);
 void rt_kprintf(const char *fmt, ...);
 #endif
 rt_int32_t rt_vsprintf(char *dest, const char *format, va_list arg_ptr);
+rt_int32_t rt_vsnprintf(char *buf, rt_size_t size, const char *fmt, va_list args);
 rt_int32_t rt_sprintf(char *buf ,const char *format, ...);
 rt_int32_t rt_snprintf(char *buf, rt_size_t size, const char *format, ...);