buildsys: use single slash version of -print-whatever

The double slash variants are not documented (doc bug?) so better use
the single slash variants like we usually do.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

Rules.mak

@@ -766,7 +766,7 @@ CFLAGS += -I$(top_srcdir)libc/sysdeps/linux/common
 CFLAGS += -I$(KERNEL_HEADERS)
 
 #CFLAGS += -iwithprefix include-fixed -iwithprefix include
-$(eval $(call cache-output-var,CC_IPREFIX,$(CC) --print-file-name=include))
+$(eval $(call cache-output-var,CC_IPREFIX,$(CC) -print-file-name=include))
 CC_INC := -isystem $(dir $(CC_IPREFIX))include-fixed -isystem $(CC_IPREFIX)
 CFLAGS += $(CC_INC)