WIN32: fix calling conventions for 32-bit builds

On 32-bit Windows Intel builds the __cdecl and __stdcall calling
conventions are different so labeling the functions that are
exported or assigned to function pointers matters.

Change-Id: I03b6f34baeb9ffb2e683fd979f12f27a5078a4da

lib/hx509/softp11.c

@@ -418,7 +418,7 @@ struct foo {
     char *id;
 };
 
-static int
+static int HX509_LIB_CALL
 add_cert(hx509_context hxctx, void *ctx, hx509_cert cert)
 {
     static char empty[] = "";