acpi: Change build log for ASL files

Currently when compiling U-Boot with ASL file, the build log says:

  ASL     board/intel/bayleybay/dsdt.c

This looks odd as ASL compiler's input is ASL file, not C file.
Change the make rule to use $< instead.
Signed-off-by: NBin Meng <bmeng.cn@gmail.com>
Reviewed-by: NStefan Roese <sr@denx.de>
Tested-by: NStefan Roese <sr@denx.de>
Reviewed-by: NSimon Glass <sjg@chromium.org>

scripts/Makefile.lib

@@ -323,7 +323,7 @@ $(obj)/%.S: $(src)/%.ttf
 
 # ACPI
 # ---------------------------------------------------------------------------
-quiet_cmd_acpi_c_asl= ASL     $@
+quiet_cmd_acpi_c_asl= ASL     $<
 cmd_acpi_c_asl=         \
 	$(CPP) -x assembler-with-cpp -P -o $<.tmp $<; \
 	iasl -p $< -tc -va $<.tmp; \