Format string mismatch in `osdp_diag.c`: `%d` used for `size_t` argument

[undici77]

Bug description
A format string mismatch in src/osdp_diag.c causes a compiler warning: the %d format specifier is used for an argument of type size_t, which leads to undefined behavior on 64-bit platforms where int and size_t differ in size (e.g., macOS, Linux x86_64).

Expected behavior
The build should complete without warnings when compiled with strict warning flags (e.g., -Wformat). The log message should correctly print the packet count using the appropriate format specifier for size_t.

Observed behavior
The compiler emits a warning:

warning: format specifies type 'int' but the argument has type 'size_t' (aka 'unsigned long') [-Wformat]
LOG_INF("Captured %d packets", num_packets);
~~           ^~~~~~~~~~~
%zu

at line 55 in src/osdp_diag.c.

Additional context

• Compiler: Apple Clang 15.0+ (and other modern compilers with -Wformat)
• Platform: macOS (64-bit), but also affects Linux/Unix 64-bit builds
• Build system: CMake with debug configuration (cmake --build ...)
• The variable num_packets is declared as size_t, but the format string incorrectly uses %d.