ctype: Fix integer type for caseconv_entry::delta

The commit 46ba1675c4 accidently changed a
bit-field from signed to unsigned.  The caseconv_entry::delta must be a
signed integer, see also "newlib/libc/ctype/caseconv.t".

Unfortunately, a standard GCC/Newlib build is done without
-Wsign-conversion.  Using this warning option would have helped to avoid
this bug:

caseconv.t:2:22: warning: unsigned conversion from 'int' to 'unsigned int:17' changes value from '-32' to '131040' [-Wsign-conversion]
   {0x0061, 25, TOUP, -32},

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>

newlib/libc/ctype/towctrans_l.c

@@ -39,7 +39,7 @@ static struct caseconv_entry {
   uint_least32_t first: 21;
   uint_least8_t diff: 8;
   uint_least8_t mode: 2;
-  uint_least32_t delta: 17;
+  int_least32_t delta: 17;
 } __attribute__ ((packed))
 caseconv_table [] = {
 #include "caseconv.t"