[client] make the compiler happy when compiling with -Wall flag

Part of the log from a OS X buildbot:
```
gcc -DHAVE_CONFIG_H -I. -I..    -DPKGDATADIR=\"/tmp/build/share/seafile\" -DPACKAGE_DATA_DIR=\""/tmp/build/share/seafile"\" -DSEAFILE_CLIENT -I../include -I../lib -I../lib -I../common -D_REENTRANT -I/tmp/build/include -I/tmp/build/include/searpc -I/opt/local/include/glib-2.0 -I/opt/local/lib/glib-2.0/include -I/opt/local/include -I/opt/local/include/glib-2.0 -I/opt/local/lib/glib-2.0/include -I/opt/local/include -D_REENTRANT -I/tmp/build/include -I/tmp/build/include/searpc -I/opt/local/include/glib-2.0 -I/opt/local/lib/glib-2.0/include -I/opt/local/include -I/opt/local/include/glib-2.0 -I/opt/local/lib/glib-2.0/include -I/opt/local/include -Wall -Wall -I/tmp/build/include -MT wt-monitor-macos.o -MD -MP -MF .deps/wt-monitor-macos.Tpo -c -o wt-monitor-macos.o wt-monitor-macos.c
wt-monitor-macos.c: In function 'stream_callback':
wt-monitor-macos.c:166: error: 'for' loop initial declaration used outside C99 mode
wt-monitor-macos.c:158: warning: unused variable 'dir'
wt-monitor-macos.c:154: warning: unused variable 'repo_id'
make[1]: *** [wt-monitor-macos.o] Error 1
make: *** [install-recursive] Error 1
```