ntdll/tests: fix a compiler warning on PowerPC

[Austin English]

Fixes http://bugs.winehq.org/show_bug.cgi?id=16750. Tested on Windows
XP, and Linux (PPC and PC x86_64).

-- 
-Austin
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diff --git a/dlls/ntdll/tests/rtlbitmap.c b/dlls/ntdll/tests/rtlbitmap.c
index fc7f6c5..d24f0af 100644
--- a/dlls/ntdll/tests/rtlbitmap.c
+++ b/dlls/ntdll/tests/rtlbitmap.c
@@ -413,7 +413,7 @@ static void test_RtlFindClearBitsAndSet(void)
 static void test_RtlFindMostSignificantBit(void)
 {
   int i;
-  CCHAR cPos;
+  signed char cPos;
   ULONGLONG ulLong;
 
   if (!pRtlFindMostSignificantBit)
@@ -442,7 +442,7 @@ static void test_RtlFindMostSignificantBit(void)
 static void test_RtlFindLeastSignificantBit(void)
 {
   int i;
-  CCHAR cPos;
+  signed char cPos;
   ULONGLONG ulLong;
 
   if (!pRtlFindLeastSignificantBit)