Fix conditional check for HAVE_DECL_SECUREZEROMEMORY

"Unlike the other `AC_CHECK_*S' macros, when a symbol is not declared,
HAVE_DECL_symbol is defined to `0' instead of leaving HAVE_DECL_symbol
undeclared. When you are sure that the check was performed,
use HAVE_DECL_symbol in #if."

Source: autoconf documentation for AC_CHECK_DECLS.

src/misc.c

@@ -53,9 +53,11 @@
 #include <sys/time.h>
 #endif
 
-#ifdef HAVE_DECL_SECUREZEROMEMORY
+#if defined(HAVE_DECL_SECUREZEROMEMORY) && HAVE_DECL_SECUREZEROMEMORY
+#ifdef HAVE_WINDOWS_H
 #include <windows.h>
 #endif
+#endif
 
 #include <stdio.h>
 #include <errno.h>
@@ -689,7 +691,7 @@ static void * (* const volatile memset_libssh)(void *, int, size_t) = memset;
 
 void _libssh2_explicit_zero(void *buf, size_t size)
 {
-#ifdef HAVE_DECL_SECUREZEROMEMORY
+#if defined(HAVE_DECL_SECUREZEROMEMORY) && HAVE_DECL_SECUREZEROMEMORY
     SecureZeroMemory(buf, size);
 #elif defined(HAVE_MEMSET_S)
     (void)memset_s(buf, size, 0, size);