Add a method to IncludeDirs to convert to string list

I'm going to use this in the rust module as well, so having a single
source of truth is useful.

mesonbuild/build.py

@@ -323,6 +323,13 @@ class IncludeDirs:
     def get_extra_build_dirs(self):
         return self.extra_build_dirs
 
+    def to_string_list(self, sourcedir: str) -> T.List[str]:
+        """Convert IncludeDirs object to a list of strings."""
+        strlist: T.List[str] = []
+        for idir in self.incdirs:
+            strlist.append(os.path.join(sourcedir, self.curdir, idir))
+        return strlist
+
 class ExtractedObjects:
     '''
     Holds a list of sources for which the objects must be extracted

mesonbuild/interpreter.py

@@ -1132,9 +1132,7 @@ class CompilerHolder(InterpreterObject):
         for i in incdirs:
             if not isinstance(i, IncludeDirsHolder):
                 raise InterpreterException('Include directories argument must be an include_directories object.')
-            for idir in i.held_object.get_incdirs():
-                idir = os.path.join(self.environment.get_source_dir(),
-                                    i.held_object.get_curdir(), idir)
+            for idir in i.held_object.to_string_list(self.environment.get_source_dir()):
                 args += self.compiler.get_include_args(idir, False)
         if not nobuiltins:
             opts = self.environment.coredata.options