Removes some minor GCC warnings

[jbrazio]

In file included from sketch\Configuration.h:55:0,

                 from sketch\Marlin.h:44,

                 from sketch\Marlin_main.cpp:35:

sketch\Marlin_main.cpp: In function 'void gcode_M200()':

sketch\macros.h:57:39: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

 #define COUNT(a) (sizeof(a)/sizeof(*a))

                                       ^

sketch\Marlin_main.cpp:5325:27: note: in expansion of macro 'COUNT'

       for (int i = 0; i < COUNT(filament_size); i++)

                           ^

sketch\Marlin_main.cpp: In function 'void calculate_volumetric_multipliers()':

sketch\macros.h:57:39: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

 #define COUNT(a) (sizeof(a)/sizeof(*a))

                                       ^

sketch\Marlin_main.cpp:8695:23: note: in expansion of macro 'COUNT'

   for (int i = 0; i < COUNT(filament_size); i++)