Remove -Wmissing-declarations warning from WARNING1 level

A function declaration declares the name of the function and the type of
the parameter it returns. A function prototype is a function declaration
that also specifies the type of the arguments of the function. Essentially,
a function prototype helps the compiler ensure whether the function call
matches the return type and the right number/type of arguments of function.
A function prototype itself serves as a function declaration for new style
functions.
The warning flag -wmissing-prototype is good enough to check for missing
function prototype and is exhaustive compared to -wmissing-declaration,
therefore  making the later redundant.

Note that, at this point, these flags are part of WARNING1 which is not
used for TF-A build by default. Several platforms use upstream libraries
(such as zlib etc) which are in old style c code. After the TF-A build
process is restructred using CMake framework, we plan to enable WARNING1,
WARNING2 and WARNING3 incrementally as the new build platform can compile
each BL binary of a particular platform with set of  unique compilation
flags.

Change-Id: I9c6bf9da74e0840e4d2624bc12376e199953c213
Signed-off-by: Justin Chadwell <justin.chadwell@arm.com>

Makefile

@@ -248,7 +248,6 @@ WARNINGS		:=	-Wall -Wmissing-include-dirs -Wunused	\
 # Additional warnings
 # Level 1
 WARNING1 := -Wextra
-WARNING1 += -Wmissing-declarations
 WARNING1 += -Wmissing-format-attribute
 WARNING1 += -Wmissing-prototypes
 WARNING1 += -Wold-style-definition