fix(makefile): use space in WARNINGS list

The tab between -Wdisabled-optimization and -Wvla is replaced with
a space. This avoids having it removed when copy/pasting the compilation
command line, and having the following error:
arm-none-eabi-gcc: error: unrecognized command line option
 '-Wdisabled-optimization-Wvla'; did you mean '-Wdisabled-optimization'?
Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I72de2a70d192a7813b1f9b55485914142d1fc428

Makefile

@@ -334,7 +334,7 @@ ASFLAGS_aarch64		=	$(march64-directive)
 
 # General warnings
 WARNINGS		:=	-Wall -Wmissing-include-dirs -Wunused	\
-				-Wdisabled-optimization	-Wvla -Wshadow	\
+				-Wdisabled-optimization -Wvla -Wshadow	\
 				-Wno-unused-parameter -Wredundant-decls
 
 # Additional warnings