refactor(libsinsp): make base transform class pure virtual

Signed-off-by: Roberto Scolaro <roberto.scolaro21@gmail.com>

Author: therealbobo

userspace/libsinsp/sinsp_filter_transformers/sinsp_filter_transformer.cpp

@@ -16,11 +16,6 @@ limitations under the License.
 
 sinsp_filter_transformer::~sinsp_filter_transformer() {}
 
-bool sinsp_filter_transformer::transform_type(ppm_param_type& t, uint32_t& flags) const {
-	throw_unsupported_err(m_type);
-	return false;
-}
-
 bool sinsp_filter_transformer::string_transformer(std::vector<extract_value_t>& vec,
                                                   ppm_param_type t,
                                                   str_transformer_func_t f) {

userspace/libsinsp/sinsp_filter_transformers/sinsp_filter_transformer.h

@@ -72,7 +72,7 @@ class sinsp_filter_transformer {
 
 	virtual ~sinsp_filter_transformer();
 
-	virtual bool transform_type(ppm_param_type& t, uint32_t& flags) const;
+	virtual bool transform_type(ppm_param_type& t, uint32_t& flags) const = 0;
 
 	virtual bool transform_values(std::vector<extract_value_t>& vals,
 	                              ppm_param_type& t,