Don't use nested namespace declarations in headers

Qt < 5.9 moc chokes on that

include/mtx/log.hpp

@@ -2,9 +2,13 @@
 
 #include <string_view>
 
-namespace mtx::utils::log {
+namespace mtx {
+namespace utils {
+namespace log {
 void
 log_warning(const std::string_view &msg);
 void
 log_error(const std::string_view &msg);
 }
+}
+}

include/mtx/pushrules.hpp

@@ -10,7 +10,8 @@
 #include <variant>
 #include <vector>
 
-namespace mtx::pushrules {
+namespace mtx {
+namespace pushrules {
 struct PushCondition
 {
         //! Required. The kind of condition to apply. See conditions for more information on the
@@ -147,3 +148,4 @@ to_json(nlohmann::json &obj, const Enabled &enabled);
 void
 from_json(const nlohmann::json &obj, Enabled &enabled);
 }
+}

include/mtx/user_interactive.hpp

@@ -8,7 +8,8 @@
 
 #include <nlohmann/json.hpp>
 
-namespace mtx::user_interactive {
+namespace mtx {
+namespace user_interactive {
 using AuthType = std::string;
 namespace auth_types {
 constexpr std::string_view password       = "m.login.password";
@@ -165,3 +166,4 @@ struct Auth
 void
 to_json(nlohmann::json &obj, const Auth &auth);
 }
+}