fix: define _DEFAULT_SOURCE and _BSD_SOURCE for BSD types

On newer glibc versions, BSD types like u_char are only available
when _DEFAULT_SOURCE or _BSD_SOURCE feature test macros are defined.

Define these macros before including any headers to ensure BSD types
are available for net-snmp headers.

c_src/towerops_nif.c

@@ -1,6 +1,10 @@
 // Pure C NIF for net-snmp MIB resolution
 // Directly links to libnetsnmp without Rust layer
 
+// Enable BSD types (u_char, u_short, etc.) on newer glibc
+#define _DEFAULT_SOURCE
+#define _BSD_SOURCE
+
 #include <erl_nif.h>
 #include <sys/types.h>
 #include <net-snmp/net-snmp-config.h>