pkg/netperf/PKGBUILD: add CFLAG -fcommon

Fix following issue:

```
/usr/bin/ld: nettest_omni.o:(.bss+0x208): multiple definition of `loc_nodelay'; nettest_bsd.o:(.bss+0x20): first defined here
/usr/bin/ld: nettest_omni.o:(.bss+0x20c): multiple definition of `rem_nodelay'; nettest_bsd.o:(.bss+0x24): first defined here
/usr/bin/ld: nettest_omni.o:(.bss+0x210): multiple definition of `loc_sndavoid'; nettest_bsd.o:(.bss+0x30): first defined here
/usr/bin/ld: nettest_omni.o:(.bss+0x214): multiple definition of `loc_rcvavoid'; nettest_bsd.o:(.bss+0x34): first defined here
/usr/bin/ld: nettest_omni.o:(.bss+0x218): multiple definition of `rem_sndavoid'; nettest_bsd.o:(.bss+0x38): first defined here
/usr/bin/ld: nettest_omni.o:(.bss+0x21c): multiple definition of `rem_rcvavoid'; nettest_bsd.o:(.bss+0x3c): first defined here
/usr/bin/ld: nettest_unix.o: in function `recv_stream_stream':
nettest_unix.c:(.text+0xd7e): warning: the use of `tempnam' is dangerous, better use `mkstemp'
collect2: error: ld returned 1 exit status
make[3]: *** [Makefile:305: netperf] Error 1
```

makepkg RT:

```
{
  "makepkg.pass": [
    1
  ]
}
```

Link: HewlettPackard/netperf#42
Signed-off-by: Zou Yu <yux.zou@intel.com>
Signed-off-by: Philip Li <philip.li@intel.com>

pkg/netperf/PKGBUILD

@@ -26,7 +26,7 @@ build()
 				--enable-burst
 	)
 
-	CFLAGS="-std=gnu89" ./configure "${configure_flags[@]}"
+	CFLAGS="-std=gnu89 -fcommon" ./configure "${configure_flags[@]}"
 	make
 }