iperf3: use -D_GNU_SOURCE to fix build error on musl

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 45103

package/network/utils/iperf3/Makefile

@@ -30,6 +30,7 @@ define Package/iperf3
   URL:=http://sourceforge.net/projects/iperf/
 endef
 
+TARGET_CFLAGS += -D_GNU_SOURCE
 CONFIGURE_ARGS += --disable-shared
 
 MAKE_FLAGS += noinst_PROGRAMS=