Add entry to CHANGELOG for supporting additional compiler warning options

Mention recent fixes for compiler warnings produced by enabling the `-Wcast-qual -Wmissing-prototypes -Wstrict-prototypes -Woverlength-strings` compiler options.

Signed-off-by: Jakub Sujak <jakub.sujak@arm.com>

Approved-by: Emil Ohlsson <emil.ohlsson@arm.com>