Replace "inline" with OPUS_INLINE.

Newer versions of MSVC are unhappy with the strategy of the build
 environment redefining "inline" (even though they don't support the
 actual keyword). Instead we define OPUS_INLINE to the right thing
 in opus_defines.h.

This is the same approach we use for restrict.
diff --git a/celt/celt_decoder.c b/celt/celt_decoder.c
index 4424b97..15ebb24 100644
--- a/celt/celt_decoder.c
+++ b/celt/celt_decoder.c
@@ -175,7 +175,7 @@
 }
 #endif /* CUSTOM_MODES */
 
-static inline opus_val16 SIG2WORD16(celt_sig x)
+static OPUS_INLINE opus_val16 SIG2WORD16(celt_sig x)
 {
 #ifdef FIXED_POINT
    x = PSHR32(x, SIG_SHIFT);