doxy: fix conditional usage, ref warnings

doxygen < 1.7.? seems to have been more tolerant of single line
\if/\endif

This change fixes warnings such as:
mainpage.dox:13: warning: unable to resolve reference to `vp8_encoder-'
for \ref command
vpx_decoder.h:193: warning: explicit link request to 'n' could not be
resolved

Change-Id: If3d04af5ede1b0d1e2c63021d0e4ac8f98db20b2

mainpage.dox

@@ -12,8 +12,12 @@
 
   This distribution of the WebM VP8 Codec SDK includes the following support:
 
-  \if vp8_encoder    - \ref vp8_encoder   \endif
-  \if vp8_decoder    - \ref vp8_decoder   \endif
+  \if vp8_encoder
+  - \ref vp8_encoder
+  \endif
+  \if vp8_decoder
+  - \ref vp8_decoder
+  \endif
 
 
   \section main_startpoints Starting Points
@@ -24,8 +28,12 @@
   - Read the \ref samples "sample code" for examples of how to interact with the
     codec.
   - \ref codec reference
-    \if encoder - \ref encoder reference \endif
-    \if decoder - \ref decoder reference \endif
+    \if encoder
+    - \ref encoder reference
+    \endif
+    \if decoder
+    - \ref decoder reference
+    \endif
 
   \section main_support Support Options & FAQ
   The WebM project is an open source project supported by its community. For