Add missing initializer braces in stb_easy_font.h

[musicinmybrain]

See the following compiler warning from gcc 11.2.1 with -Wall:

In file included from test_easyfont.c:1:
../stb_easy_font.h: In function 'stb_easy_font_print':
../stb_easy_font.h:205:29: warning: missing braces around initializer [-Wmissing-braces]
  205 |     stb_easy_font_color c = { 255,255,255,255 }; // use structure copying to avoid needing depending on memcpy()
      |                             ^
      |                               {               }

There should be one set of braces for the structure stb_easy_font_color, and then another to contain the elements of the array member c. This is a pedantic fix, as typical compilers will compile it correctly and will at most warn rather than erroring.

No contributors list entry is needed for this trivial contribution.