g.mapsets: add missing declared or defined argument type

[nilason]

Add missing declaration/definition of argument type.

Log extract:

/usr/bin/clang -I/opt/local/include -isystem/opt/local/include -isystem/opt/local/include/libomp -std=c11 -I/opt/local/include -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX13.sdk -arch arm64 -O0 --debug -Wall -Wextra -Wpedantic -Wno-error=deprecated-non-prototype -D_FORTIFY_SOURCE=3 -Wvla -Wno-error=strict-prototypes -DGL_SILENCE_DEPRECATION   -I/dev/grass/dist.aarch64-apple-darwin23.4.0/include -I/dev/grass/dist.aarch64-apple-darwin23.4.0/include    -DPACKAGE=\""grassmods"\"   -I/dev/grass/dist.aarch64-apple-darwin23.4.0/include -I/dev/grass/dist.aarch64-apple-darwin23.4.0/include -DRELDIR=\"general/g.mapsets\" -o OBJ.aarch64-apple-darwin23.4.0/get_maps.o -c get_maps.c
In file included from get_maps.c:5:
./local_proto.h:9:34: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
void list_accessible_mapsets_json();
                                 ^
                                  void
1 warning generated.

[echoix]

Do you happen to have a bit more time than me for the Windows CI? I'm heading to a world championship at the end of next week, and have practices almost each day.. I'll be loose only at the end of the month, and be there at the community meeting ;)