ecore_drm: make sure we include math.h and link against libm for floor()

When we introduced the usage of floor() in ecore_drm_evdev.c we forgot to
include math.h for it as well as linking against libm. This caused linking
failures on some systems while it still worked on others. Better fix it for
good.

Original patch by aerodynamik.

ref T2953

configure.ac

@@ -3472,6 +3472,8 @@ EFL_INTERNAL_DEPEND_PKG([ECORE_DRM], [eina])
 EFL_DEPEND_PKG([ECORE_DRM], [DRM], [libdrm >= 2.4 xkbcommon >= 0.3.0 gbm])
 EFL_DEPEND_PKG([ECORE_DRM], [LIBINPUT], [libinput >= 0.6.0])
 
+EFL_ADD_LIBS([ECORE_DRM], [-lm])
+
 # API change from 0.7 to 0.8. So we define this to support both for now.
 PKG_CHECK_EXISTS([libinput >= 0.8.0],
                  [have_libinput_new="yes"],

src/lib/ecore_drm/ecore_drm_evdev.c

@@ -4,6 +4,7 @@
 
 #include "ecore_drm_private.h"
 #include <ctype.h>
+#include <math.h>
 
 static void  _device_modifiers_update(Ecore_Drm_Evdev *edev);