utils: fix -Wsign-compare in definition of OSMO_STRBUF_CHAR_COUNT

We're seeing tons of -Wsign-compare warnings since I merged 0f59cebf:

include/osmocom/core/utils.h: In function 'size_t _osmo_strbuf_char_count(const osmo_strbuf*)':
include/osmocom/core/utils.h:24:29: error: comparison of integer expressions of different
                                    signedness: 'long int' and 'long unsigned int'
                                    [-Werror=sign-compare]
   24 | #define OSMO_MIN(a, b) ((a) >= (b) ? (b) : (a))
      |                         ~~~~^~~~~~
include/osmocom/core/utils.h:309:16: note: in expansion of macro 'OSMO_MIN'
  309 |         return OSMO_MIN(sb->pos - sb->buf, sb->len - 1);
      |                ^~~~~~~~

Interestingly enough, this -Wsign-compare problem has always been the
case, even before commit 0f59cebf.  And somehow this did not show up
when building libosmocore.git, but only when building C++ projects
(osmo-pcu and osmo-trx).

Perhaps it has something to do with how g++ compiles extern "C" code.

Change-Id: I8e396459409e4260b8715f9e890e8972d4609a31
Fixes: 0f59cebf "utils: improve readability of OSMO_STRBUF_CHAR_COUNT"
---
M include/osmocom/core/utils.h
1 file changed, 30 insertions(+), 1 deletion(-)