First thing is to remove your alterations from the "usbdrv.h" file.
Second, you could try what aarg suggests. ie add 'const' to the beginning to each of the offending lines in the library files.
....10 minutes later....
I just did that, and got it all to compile OK with the provided example.
(I compiled for UNO, with IDE V1.6.5)
Three files need to be modified:-
"usbdrv.h"
"usbdrv.c"
"UsbKeyboard.h"
There are no changes needed to the "Shrimpkey.ino" or "settings.h" files, just the above-mentioned files from the "UsbKeyboard" library.
In the three files I listed above, everywhere that there's a "PROGMEM", you need to add "const" before the variable type name.
Or, since I've already done it once, you could back up your current library files just in case, then replace them with the three modified ones that I've attached at the bottom of this post.
UsbKeyboard.h (6.42 KB)
usbdrv.c (24.1 KB)
usbdrv.h (30.5 KB)