include/ntsecapi.h: Add missing `__stdcall` specifiers.

This commit adds `__stdcall` to the following functions:

    RtlGenRandom()
    RtlEncryptMemory()
    RtlDecryptMemory()

Absence of these specifiers used to cause undefined references to
`SystemFunction036` when compiling the following code for x86:

    #include <minwindef.h>
    #include <ntsecapi.h>

    int main(void)
      {
        char buffer[100];
        RtlGenRandom(buffer, sizeof(buffer));
      }

Signed-off-by: Liu Hao <lh_mouse@126.com>