Reset letter spacing for form elements

Some browsers, such as Chrome, set `letter-spacing: normal` for form elements. For consistency with the other styles, it should be set to `inherit` by Preflight.

src/css/preflight.css

@@ -175,6 +175,7 @@ textarea {
   font-size: 100%; /* 1 */
   font-weight: inherit; /* 1 */
   line-height: inherit; /* 1 */
+  letter-spacing: inherit; /* 1 */
   color: inherit; /* 1 */
   margin: 0; /* 2 */
   padding: 0; /* 3 */