kbuild: Fix soundmodem/Makefile

gentbl is a program that generates some header files. The recent kbuild
changes have the "interesting" effect that this now outputs the header
files to the root directory of the kernel tree instead of
drivers/net/hamradio/soundmodem ...

The following patch fixes this breakage:

drivers/net/hamradio/soundmodem/Makefile

@@ -38,5 +38,5 @@ $(obj)/sm_hapn4800.o:   $(obj)/sm_tbl_hapn4800.h
 $(obj)/sm_fsk9600.o:    $(obj)/sm_tbl_fsk9600.h
 
 $(obj)/sm_tbl_%: $(obj)/gentbl
-	$(obj)/gentbl
+	cd $(obj) && ./gentbl