Fix -Wundef -Wstrict-prototypes

[prj-]

src/petsc4py/PETSc.c:22:5: warning: 'CYTHON_LIMITED_API' is not defined, evaluates to 0 [-Wundef]
#if CYTHON_LIMITED_API
    ^
src/petsc4py/PETSc.c:5283:47: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
static unsigned long __Pyx_get_runtime_version();
                                              ^
                                               void
src/petsc4py/PETSc.c:561483:49: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
  static unsigned long __Pyx_get_runtime_version() {
                                                ^
                                                 void
3 warnings generated.

[prj-]

@matusvalo, if you think my changes will effectively address the warnings reported in the PR description (I have no idea myself), could you please get this integrated?

[scoder]

Looks right. Thanks.