Missing an explicit include of <mach-o/dyld.h> on Apple platforms. (#932)

On some Apple platforms (e.g. iOS), the set of includes we have in
`_TestingInternals` doesn't transitively include dyld.h, causing a build
failure when building for those platforms. So explicitly include it.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/_TestingInternals/include/Includes.h

@@ -127,6 +127,10 @@
 #if !SWT_NO_LIBDISPATCH
 #include <dispatch/dispatch.h>
 #endif
+
+#if !SWT_NO_DYNAMIC_LINKING
+#include <mach-o/dyld.h>
+#endif
 #endif
 
 #if defined(__FreeBSD__)