Fix compiling System.Net.Security.Native with HAVE_HEIMDAL_HEADERS (#54682)

We are using malloc/free but don't declare a header. It likely happens to be included from GSS/GSS.h which is why it builds now, but not from the Heimdal headers.

Author: MichalStrehovsky

src/libraries/Native/Unix/System.Net.Security.Native/pal_gssapi.c

@@ -19,6 +19,7 @@
 
 #include <assert.h>
 #include <string.h>
+#include <stdlib.h>
 
 #if defined(GSS_SHIM)
 #include <dlfcn.h>