genhtml has been generating the following warning with this new code:
WARNING: function data mismatch at /path/src/common/unicode_norm.c:102

HTML coverage reports care about the uniqueness of functions defined in
source files, ignoring any assumptions around CFLAGS.  783f0cc6
introduced a duplicated definition of get_code_entry(), leading to a
warning and potentially some incorrect data generated in the reports.
This refactors the code so as the code has only one function
declaration, fixing the warning.

Oversight in 783f0cc6.

Reported-by: Tom Lane
Author: Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/207789.1603469272@sss.pgh.pa.us