libraw/dc_raw: fix gcc-6 failures

With gcc-6, it is no longer allowed to narrow the type of constants in
a constant array declaration.

Fixes:
    http://autobuild.buildroot.org/results/081/0811531872f69f9febbdc482dfbdd7fb5c35d1c8/build-end.log

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

diff -durN a/Source/LibRawLite/internal/dcraw_common.cpp b/Source/LibRawLite/internal/dcraw_common.cpp
--- a/Source/LibRawLite/internal/dcraw_common.cpp
+++ b/Source/LibRawLite/internal/dcraw_common.cpp
@@ -2479,7 +2479,7 @@
 
 void CLASS kodak_radc_load_raw()
 {
-  static const char src[] = {
+  static const signed char src[] = {
     1,1, 2,3, 3,4, 4,2, 5,7, 6,5, 7,6, 7,8,
     1,0, 2,1, 3,3, 4,4, 5,2, 6,7, 7,6, 8,5, 8,8,
     2,1, 2,3, 3,0, 3,2, 3,4, 4,6, 5,5, 6,7, 6,8,
@@ -4519,7 +4519,7 @@
  */
 void CLASS vng_interpolate()
 {
-  static const signed char *cp, terms[] = {
+  static const int *cp, terms[] = {
     -2,-2,+0,-1,0,0x01, -2,-2,+0,+0,1,0x01, -2,-1,-1,+0,0,0x01,
     -2,-1,+0,-1,0,0x02, -2,-1,+0,+0,0,0x03, -2,-1,+0,+1,1,0x01,
     -2,+0,+0,-1,0,0x06, -2,+0,+0,+0,1,0x02, -2,+0,+0,+1,0,0x03,