Avoid implicit cast from size_t to uint32_t

[travbid]

Hello, we have an error on line 42 of path_from_handle.hpp.
With the XCode 14.1 update, the -Wshorten-64-to-32 flag has somehow been enabled in our project and shows this warning when including #include "boost/dll/import.hpp"

In file included from ***/boost/dll/import.hpp:16:
In file included from ***/boost/dll/shared_library.hpp:26:
In file included from ***/boost/dll/detail/posix/shared_library_impl.hpp:13:
***/boost/dll/detail/posix/path_from_handle.hpp:42:59: error: implicit conversion loses integer precision: 'std::size_t' (aka 'unsigned long') to 'uint32_t' (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
const char* image_name = _dyld_get_image_name(i);
~~~~~~~~~~~~~~~~~~~~ ^
1 error generated.

According to the linked docs on line 37, _dyld_get_image_name() takes a uint32_t but here is passed a size_t. With -Wconversion enabled this creates a warning when size_t is a larger type than uint32_t.
_dyld_image_count() returns a uint32_t so I think this can be solved by not casting to size_t in the first place.

Edit: I removed the std:: prefix because mach-o/dyld.h has the type un-prefixed and it seems uint32_t is not part of the C++03 standard.

[apolukhin]

Many thanks for the PR!