Makefile: reintroduce KDIR variable definition, as a default

Necesarry as default when make is called without dkms.
If called with dkms, luckily the variable and value passed as command
line parameter overides the one defined in the makefile as desired. So
removing was not necesarry, details in:

https://www.gnu.org/software/make/manual/make.html#Using-Variables

Makefile

@@ -3,6 +3,7 @@ obj-m := ./src/tuxedo_keyboard.o
 tuxedo_tuxedo-objs := ./src/tuxedo_keyboard.o
 
 PWD := $(shell pwd)
+KDIR := /lib/modules/$(shell uname -r)/build
 
 all:
 	make -C $(KDIR) M=$(PWD) modules