wmaker: Cleaned dangerous function prototype usage

- remove extern declaration in source file, use header instead

- add inclusion of header defining the functions of the file to
get the compiler to cross-check them

- marked static the functions that should not be visible ouside
their file

src/placement.h

@@ -33,6 +33,8 @@ int calcIntersectionArea(int x1, int y1, int w1, int h1, int x2, int y2, int w2,
 
 void PlaceWindow(WWindow *wwin, int *x_ret, int *y_ret, unsigned width, unsigned height);
 
+void InteractivePlaceWindow(WWindow * wwin, int *x_ret, int *y_ret, unsigned width, unsigned height);
+
 /* Set the points x and y inside the screen */
 void get_right_position_on_screen(WScreen *scr, int *x, int *y, int size_x, int size_y);