jtag: drivers: build usb_blaster only if enabled

Fix build failure when libusb dev files are not available and
so usb_blaster is disabled.

Change-Id: I4cda7df689cdb4b62b733cbbab813241cf561e29
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3920
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

src/jtag/drivers/Makefile.am

@@ -78,6 +78,7 @@ DRIVERFILES += %D%/jtag_vpi.c
 endif
 if USB_BLASTER_DRIVER
 %C%_libocdjtagdrivers_la_LIBADD += %D%/usb_blaster/libocdusbblaster.la
+include %D%/usb_blaster/Makefile.am
 endif
 if AMTJTAGACCEL
 DRIVERFILES += %D%/amt_jtagaccel.c
@@ -171,4 +172,3 @@ DRIVERHEADERS = \
 	%D%/versaloon/versaloon_include.h \
 	%D%/versaloon/versaloon_internal.h
 
-include %D%/usb_blaster/Makefile.am