Emacs 26 warning about no previous prototype for 'surrogates_to_codepoint'

[martin rudalics]

When compiling xdisp.c for Emacs 26.2 I currently see under MinGW (GCC 4.8.1)

  CC       xdisp.o
In file included from ../../src/keyboard.h:23:0,
                 from ../../src/xdisp.c:313:
../../src/coding.h:680:1: warning: no previous prototype for 
'surrogates_to_codepoint' [-Wmissing-prototypes]
 surrogates_to_codepoint (int low, int high)
 ^

I'm somewhat lost with this.

martin