Set BMAP_BYTE to unsigned char always

[kazarmy]

This pr sets BMAP_BYTE to unsigned char always for the following reasons:

1. Bitmap bytes are bytes and not characters.
2. C++ does have an unsigned char type (https://en.cppreference.com/w/cpp/language/types#Character_types).
3. The current code is causing compilation errors on modern Ubuntu C++ compilers. E.g. with g++ 9.3.0:

   g++ -c -g -O2 -fpermissive -Wno-write-strings -g    -DHAVE_ACDEFS_H -I. -I./.. -I./../kernel -I/usr/include/tcl8.6 -I/usr/include/tcl8.6     tkinit.c
   In file included from tkinit.c:51:
   ./../bitmaps/lookglass.b:8:50: error: narrowing conversion of ‘248’ from ‘int’ to ‘char’ [-Wnarrowing]
   8 |    0x00, 0x0c, 0x00, 0x18, 0x00, 0x30, 0x00, 0x20};
     |

   and with clang++ 10.0:

   clang++ -c -g -O2 -fpermissive -Wno-write-strings -g    -DHAVE_ACDEFS_H -I. -I./.. -I./../kernel -I/usr/include/tcl8.6 -I/usr/include/tcl8.6     tkinit.c
   clang: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
   In file included from tkinit.c:51:
   ./../bitmaps/lookglass.b:6:4: error: constant expression evaluates to 248 which cannot be narrowed to type 'char' [-Wc++11-narrowing]
   0xf8, 0x00, 0x04, 0x01, 0x02, 0x02, 0x01, 0x04, 0x01, 0x04, 0x01, 0x04,
   ^~~~