Suppress fopen deprecated warning when building ELF2UF2

[kkitayam]

When building elf2uf2 with clang x86_64-pc-windows-msvc and Windows 10 SDK, the following warning occurs. This PR suppresses the warnings.

[1/2] Building CXX object CMakeFiles/elf2uf2.dir/main.cpp.obj
C:/pico-sdk/tools/elf2uf2/main.cpp:359:16: warning: 'fopen' is deprecated: This function or variable may be unsafe. Consider using fopen_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details. [-Wdeprecated-declarations]
    FILE *in = fopen(in_filename, "rb");
               ^
C:\Program Files (x86)\Windows Kits\10\Include\10.0.17763.0\ucrt\stdio.h:207:20: note: 'fopen' has been explicitly marked deprecated here
    _Check_return_ _CRT_INSECURE_DEPRECATE(fopen_s)
                   ^
C:\Program Files (x86)\Microsoft Visual Studio\2017\WDExpress\VC\Tools\MSVC\14.16.27023\include\vcruntime.h:255:55: note: expanded from macro '_CRT_INSECURE_DEPRECATE'
        #define _CRT_INSECURE_DEPRECATE(_Replacement) _CRT_DEPRECATE_TEXT(    \
                                                      ^
C:\Program Files (x86)\Microsoft Visual Studio\2017\WDExpress\VC\Tools\MSVC\14.16.27023\include\vcruntime.h:245:47: note: expanded from macro '_CRT_DEPRECATE_TEXT'
#define _CRT_DEPRECATE_TEXT(_Text) __declspec(deprecated(_Text))
                                              ^

Environment details:

clang version 14.0.6 Target: x86_64-pc-windows-msvc Thread model: posix InstalledDir: C:\Program Files\LLVM\bin

Windows SDK 10.0.17763.0 Microsoft Visual Studio 2017