wrapper: make inclusion of Windows csprng header tightly scoped

wrapper: make inclusion of Windows csprng header tightly scoped

Including NTSecAPI.h in git-compat-util.h causes build errors in any
other file that includes winternl.h. NTSecAPI.h was included in order to
get access to the RtlGenRandom cryptographically secure PRNG. This
change scopes the inclusion of ntsecapi.h to wrapper.c, which is the only
place that it's actually needed.

The build breakage is due to the definition of UNICODE_STRING in
NtSecApi.h:
    #ifndef _NTDEF_
    typedef LSA_UNICODE_STRING UNICODE_STRING, *PUNICODE_STRING;
    typedef LSA_STRING STRING, *PSTRING ;
    #endif

LsaLookup.h:
    typedef struct _LSA_UNICODE_STRING {
        USHORT Length;
        USHORT MaximumLength;
    #ifdef MIDL_PASS
        [size_is(MaximumLength/2), length_is(Length/2)]
    #endif // MIDL_PASS
        PWSTR  Buffer;
    } LSA_UNICODE_STRING, *PLSA_UNICODE_STRING;

winternl.h also defines UNICODE_STRING:
    typedef struct _UNICODE_STRING {
        USHORT Length;
        USHORT MaximumLength;
        PWSTR  Buffer;
    } UNICODE_STRING;
    typedef UNICODE_STRING *PUNICODE_STRING;

Both definitions have equivalent layouts. Apparently these internal
Windows headers aren't designed to be included together. This is
an oversight in the headers and does not represent an incompatibility
between the APIs.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>