fix(makefile): use space in WARNINGS list
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 <e753f6969c288905606e3c07464886d4f1fc1be0@foss.st.com>
Change-Id: I72de2a70d192a7813b1f9b55485914142d1fc428

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware