jtag/drivers/cmsis_dap: fix build with gcc 10.1.0

Avoid multiple definition of cmsis_dap_usb_backend and
cmsis_dap_hid_backend using 'extern'.
Move the prototypes in cmsis_dap.h.
Remove the useless #if/#endif around the prototypes.

Change-Id: I8d73fe148e2155620244bc887d4235e9af530e30
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5790
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>