tests: fix compilation with modern compilers

GCC 14 and clang 16, after 20+ years, finally enforce c99 language
requirements. Previously, to keep old code compiling, the use of K&R C
constructs was simply allowed despite the standard, and so users could
accidentally use those constructs without realizing it or being warned.

Now, the compiler fatally errors out when encountering the problematic
code. This can also be observed in older versions of the compiler by
using the following flags:

```
-Werror=implicit-function-declaration -Werror=implicit-int -Werror=int-conversion -Werror=incompatible-pointer-types
```

In hamlib's case, the use of a function without declaring it is illegal.
It's also a very simple fix though -- the standard header which declared
the stdlib exit() function was forgotten and never included. Simply add
the missing include, and make the source code automatically compliant.

Bug: https://bugs.gentoo.org/923108

tests/memload.c

@@ -18,6 +18,8 @@
  *
  */
 
+#include <stdlib.h>
+
 #include <hamlib/rig.h>
 
 #ifdef HAVE_XML2