Added search path relative to running binary on Windows.

In contrast to *IX it doesn't make much sense to add compile time defined
search paths to Windows binaries: There's no standard path like /usr/local/bin
(and there are no symbolic links to link from there to another location).

On the other hand it's (again in contrast to *IX) easy for Windows binaries
to determine their own paths. Therefore it's appropriate to make use of that
to add run time defined default search paths.

src/ca65/incpath.c

@@ -87,6 +87,9 @@ void FinishIncludePaths (void)
 #ifdef CA65_INC
     AddSearchPath (IncSearchPath, STRINGIZE (CA65_INC));
 #endif
+
+    /* Add paths relative to the parent directory of the Windows binary. */
+    AddSubSearchPathFromWinBin (IncSearchPath, "asminc");
 }