Makefile: Adapt $(space) definition

GNU Make 4.3 is more picky about the $(space) definition. It seems that
the variable ends up being empty.

Change-Id: If07db29fc63e8a45e9b0869df894ce3420fa9b95
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/libhwbase/+/39163
Tested-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Nico Huber <nico.h@gmx.de>
diff --git a/Makefile b/Makefile
index a25deb6..14a7d4f 100644
--- a/Makefile
+++ b/Makefile
@@ -22,7 +22,7 @@
 $(binary):
 
 space :=
-space +=
+space := $(space) $(space)
 comma := ,
 
 strip_quotes = $(strip $(subst ",,$(1)))