build(bif-coverage): add stdlib unit path so it builds by default

The tool is now activeByDefault, so it is reached by a plain
'pasbuild compile'. With manualUnitPaths=true it only searches the listed
paths, which lacked the stdlib directory where SysUtils and the other
units it uses live, so the default build failed with 'Unit SysUtils not
found'. Add ../../stdlib/src/main/pascal, mirroring the compiler module's
own stdlib unit path.

tools/bif-coverage/project.xml

@@ -11,6 +11,8 @@
     <unitPaths>
       <path>src/main/pascal</path>
       <path>../../compiler/src/main/pascal</path>
+      <!-- SysUtils and other stdlib units the analyser uses. -->
+      <path>../../stdlib/src/main/pascal</path>
     </unitPaths>
   </build>
 </project>