Fix build using MinGW.

I tried to build json-c using a MinGW cross compiler, but it fails:

    .../random_seed.c:184:0: error: ignoring #pragma comment  [-Werror=unknown-pragmas]
    .../random_seed.c: In function 'get_cryptgenrandom_seed':
    .../random_seed.c:190:5: error: unknown type name 'HCRYPTPROV'
    .../random_seed.c:193:5: error: implicit declaration of function 'CryptAcquireContextW' [-Werror=implicit-function-declaration]
    .../random_seed.c:193:49: error: 'PROV_RSA_FULL' undeclared (first use in this function)
    .../random_seed.c:193:49: note: each undeclared identifier is reported only once for each function it appears in
    .../random_seed.c:193:64: error: 'CRYPT_VERIFYCONTEXT' undeclared (first use in this function)
    .../random_seed.c:193:86: error: 'CRYPT_SILENT' undeclared (first use in this function)
    .../random_seed.c:198:5: error: implicit declaration of function 'CryptGenRandom' [-Werror=implicit-function-declaration]
    .../random_seed.c:203:5: error: implicit declaration of function 'CryptReleaseContext' [-Werror=implicit-function-declaration]

There are two problems here -- a missing header, and an unsupported pragma -- and this patch fixes both.

The library now appears to work fine, in my use-case, although I'm not sure if that exercises this specific code.