bitstream: swab: Fix potentially undefined shifts.

Index: libdca/bitstream.h
--- libdca/bitstream.h.orig
+++ libdca/bitstream.h
@@ -30,8 +30,10 @@
 #else
 
 #   define swab32(x)\
-((((uint8_t*)&x)[0] << 24) | (((uint8_t*)&x)[1] << 16) |  \
- (((uint8_t*)&x)[2] << 8)  | (((uint8_t*)&x)[3]))
+    (((uint32_t)(((uint8_t*)&x)[0]) << 24u) | \
+     ((uint32_t)(((uint8_t*)&x)[1]) << 16u)  | \
+     ((uint32_t)(((uint8_t*)&x)[2]) << 8u)   | \
+     ((uint32_t)(((uint8_t*)&x)[3])))
 
 #endif