gdb: add declaration to Python init function

gdb: add declaration to Python init function

When I try to enable -Wmissing-declarations, I get this error:

      CXX    python/python.o
    /home/smarchi/src/binutils-gdb/gdb/python/python.c: In function ‘PyObject* init__gdb_module()’:
    /home/smarchi/src/binutils-gdb/gdb/python/python.c:1582:1: error: no previous declaration for ‘PyObject* init__gdb_module()’ [-Werror=missing-declarations]
     init__gdb_module (void)
     ^~~~~~~~~~~~~~~~

Prevent it by providing a declaration just before the definition.

gdb/ChangeLog:

	* python/python.c (init__gdb_module): Add declaration.

Change-Id: I394bc691b7db624708cc4cb2cda28a56ab85a82b