rg_etc: use void to force empty function parameters

We have to use void in a function declaration if we want no function
parameters. Using just empty parenthesis means the function takes an
unspecified number of parameters.

We had it correct for most declarations and this series fixes it for
the rest.

src/static_libs/rg_etc/rg_etc1.h

@@ -28,7 +28,7 @@ typedef struct
 
 // pack_etc1_block_init() should be called before calling pack_etc1_block(),
 // Otherwise rg_etc1_pack_block() will call rg_etc1_pack_block_init() for you.
-void rg_etc1_pack_block_init();
+void rg_etc1_pack_block_init(void);
 
 // Packs a 4x4 block of 32bpp BGRA pixels to an 8-byte ETC1 block.
 // 32-bit BGRA pixels must always be arranged as (B,G,R,A) (B first, A last) in memory, independent of platform endianness. A should always be 255.