fix: Unbreak build on CentOS 6. (#1040)

The fix eliminates a duplicate definition of the iperf_size_t datatype
when both iperf.h and iperf_api.h are included.

Fixes #1039.

src/iperf.h

@@ -68,7 +68,9 @@
 #include <openssl/evp.h>
 #endif // HAVE_SSL
 
+#if !defined(__IPERF_API_H)
 typedef uint64_t iperf_size_t;
+#endif // __IPERF_API_H
 
 struct iperf_interval_results
 {

src/iperf_api.h

@@ -45,7 +45,9 @@ struct iperf_interval_results;
 struct iperf_stream;
 struct iperf_time;
 
+#if !defined(__IPERF_H)
 typedef uint64_t iperf_size_t;
+#endif // __IPERF_H
 
 /* default settings */
 #define Ptcp SOCK_STREAM