[MSPAINT] Don't use SIZEOF on a pointer

[MSPAINT] Don't use SIZEOF on a pointer

Fixes GCC 8 warning:
base/applications/mspaint/definitions.h:16:31: error: division 'sizeof (LPWSTR {aka wchar_t*}) / sizeof (WCHAR {aka wchar_t})' does not compute the number of array elements [-Werror=sizeof-pointer-div]
 #define SIZEOF(a)  (sizeof(a) / sizeof((a)[0]))
                     ~~~~~~~~~~^~~~~~~~~~~~~~~~
base/applications/mspaint/main.cpp:134:55: note: in expansion of macro 'SIZEOF'
                 lstrcpyn(pon->lpOFN->lpstrFile, Path, SIZEOF(pon->lpOFN->lpstrFile));
                                                       ^~~~~~