Use "unsigned char:1" for flags

[pengi]

"char" is treated, at least in many cases, as signed values. Having that field as 1 bit means it can be treated as one sign bit, no data bits. Thus only values 0 and -1 will be accepted

It seems to differ between versions of the compiler. But in any case, explicitly defining it as unsigned means only the values 0 and 1 will be accepted, and therefore there shouldn't be any issues between compiler versions.

This is a fix for an issue I had using:

gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0

Was compiling with flags:

gcc -g -O3 -Wdouble-promotion -Wall -Werror -Wpedantic

And the errors I got was:

src/vendor/tinyprintf/tinyprintf.c: In function ‘tfp_format’:
src/vendor/tinyprintf/tinyprintf.c:282:36: error: overflow in conversion from ‘int’ to ‘signed char:1’ changes value from ‘1’ to ‘-1’ [-Werror=overflow]
  282 |                     p.align_left = 1;
      |                                    ^
src/vendor/tinyprintf/tinyprintf.c:285:28: error: overflow in conversion from ‘int’ to ‘signed char:1’ changes value from ‘1’ to ‘-1’ [-Werror=overflow]
  285 |                     p.lz = 1;
      |                            ^
src/vendor/tinyprintf/tinyprintf.c:288:29: error: overflow in conversion from ‘int’ to ‘signed char:1’ changes value from ‘1’ to ‘-1’ [-Werror=overflow]
  288 |                     p.alt = 1;
      |                             ^
src/vendor/tinyprintf/tinyprintf.c:305:22: error: overflow in conversion from ‘int’ to ‘signed char:1’ changes value from ‘1’ to ‘-1’ [-Werror=overflow]
  305 |               p.lz = 1;  /* zero-padding */
      |                      ^
src/vendor/tinyprintf/tinyprintf.c:374:25: error: overflow in conversion from ‘int’ to ‘signed char:1’ changes value from ‘1’ to ‘-1’ [-Werror=overflow]
  374 |                 p.alt = 1;
      |                         ^

This patch should solve it.

I've been trying to also add the corresponding warning to the make file, but can't seem to reproduce it in tinyprintfs build environment.