Fixed compiler warnings emitted by GCC 4.7.2

Hello.

When I compiled wmidump with GCC 4.7.2 on Arch Linux I got several warnings:

[david@tantalum wmidump]$ make
gcc  -O2 -ggdb3 -Wall -W -pedantic -std=gnu99 -D_GNU_SOURCE -c wmidump.c
wmidump.c:57:4: warning: ISO C99 doesn’t support unnamed structs/unions [-pedantic]
wmidump.c:58:3: warning: ISO C99 doesn’t support unnamed structs/unions [-pedantic]
wmidump.c: In function ‘read_wdg’:
wmidump.c:261:15: warning: pointer of type ‘void *’ used in arithmetic [-pedantic]
wmidump.c: In function ‘main’:
wmidump.c:273:14: warning: unused parameter ‘argc’ [-Wunused-parameter]
wmidump.c:273:26: warning: unused parameter ‘argv’ [-Wunused-parameter]
wmidump.c: At top level:
wmidump.c:108:13: warning: ‘wmi_swap_bytes’ defined but not used [-Wunused-function]
wmidump.c:134:13: warning: ‘wmi_parse_guid’ defined but not used [-Wunused-function]
gcc -O2 -ggdb3 -Wall -W -pedantic -std=gnu99 -D_GNU_SOURCE  -o wmidump wmidump.o

This pull request fixes all of those warnings.

By the way, I saw your very nice [tutorial about how to write a WMI driver](http://lwn.net/Articles/391230/).  I am trying to get the backlight hotkeys on my ASUS G75VW-DH72 notebook to work and I will probably submit a pull request to acpi4asus-dkms when I figure it out; I think I will need to change something in asus-nb-wmi.  Thanks for all your work to support ASUS notebooks!

--David