Set files prefix manually to override autodiscover by genhtml.

Sometimes genhtml incorrectly discovers the common path, e.g.
linux-test-project/lcov#130
so it is beter to point it manually to the proper location.
wq

Author: rlalik

code-coverage.cmake

@@ -649,7 +649,7 @@ function(add_code_coverage_all_targets)
       add_custom_target(
         ccov-all
         COMMAND ${GENHTML_PATH} -o ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged
-                ${COVERAGE_INFO}
+                ${COVERAGE_INFO} -p ${CMAKE_SOURCE_DIR}
         DEPENDS ccov-all-capture)
 
     endif()