context: constify pa_context_set_error()

This allows constifying public API functions that report their errors
via the context error but don't modify the context in any other way.
Philosophical arguments could be made why this is wrong, but I believe
in practice this is a net positive change.

src/pulse/context.c

@@ -313,7 +313,7 @@ void pa_context_set_state(pa_context *c, pa_context_state_t st) {
     pa_context_unref(c);
 }
 
-int pa_context_set_error(pa_context *c, int error) {
+int pa_context_set_error(const pa_context *c, int error) {
     pa_assert(error >= 0);
     pa_assert(error < PA_ERR_MAX);