CLEANUP: debug: mark the RNG's seed as unsigned

Since commit 8a069eb9a ("MINOR: debug: add a trivial PRNG for scheduler
stress-tests"), 32-bit gcc 4.7 emits this warning when parsing the
initial seed for the debugger's RNG (2463534242):

  src/debug.c:46:1: warning: this decimal constant is unsigned only in ISO C90 [enabled by default]

Let's mark it explicitly unsigned.
diff --git a/src/debug.c b/src/debug.c
index f83a7c0..460ee4d 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -43,10 +43,9 @@
 unsigned int debug_commands_issued = 0;
 
 /* Xorshift RNGs from http://www.jstatsoft.org/v08/i14/paper */
-static THREAD_LOCAL unsigned int y = 2463534242;
+static THREAD_LOCAL unsigned int y = 2463534242U;
 static unsigned int debug_prng()
 {
-
         y ^= y << 13;
         y ^= y >> 17;
         y ^= y << 5;