matchbox-panel: fixed undefined reference to `log' and `lrint'

Encountered the following while compiling matchbox-panel

mb-applet-wireless.o: In function `update_wireless':
.. matchbox-panel-0.9.3/applets/mb-applet-wireless.c:143: undefined reference to `log'
.. matchbox-panel-0.9.3/applets/mb-applet-wireless.c:143: undefined reference to `lrint'

fixed the issue by adding
1) #include <math.h> in mb-applet-wireless.c
2) -lm option to WIRELESS_LIB in applets/Makefile.in

Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[arnout: rename patch and add description and Signed-off-by]

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>