pyBigWig.c: fix build failure with gcc 14.

[emollier]

As initially identified in Debian bug #1075407, pyBigWig fails to build with gcc 14 with the following relevant output:

pyBigWig.c:773:12: warning: return discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  773 |     return PyUnicode_AsUTF8(obj);
      |            ^~~~~~~~~~~~~~~~~~~~~
pyBigWig.c: In function ‘pyBwAddHeader’:
pyBigWig.c:897:32: error: passing argument 1 of ‘bwCreateChromList’ from incompatible pointer type [-Wincompatible-pointer-types]
  897 |     bw->cl = bwCreateChromList(chroms, lengths, n);
      |                                ^~~~~~
      |                                |
      |                                char **
In file included from pyBigWig.h:3,
                 from pyBigWig.c:3:
/usr/include/bigWig.h:497:51: note: expected ‘const char * const*’ but argument is of type ‘char **’
  497 | chromList_t *bwCreateChromList(const char* const* chroms, const uint32_t *lengths, int64_t n);
      |                                ~~~~~~~~~~~~~~~~~~~^~~~~~
pyBigWig.c: In function ‘PyAddIntervals’:
pyBigWig.c:1251:29: error: passing argument 2 of ‘bwAddIntervals’ from incompatible pointer type [-Wincompatible-pointer-types]
 1251 |     rv = bwAddIntervals(bw, cchroms, ustarts, uends, fvalues, n);
      |                             ^~~~~~~
      |                             |
      |                             char **
/usr/include/bigWig.h:524:57: note: expected ‘const char * const*’ but argument is of type ‘char **’

This is because starting with gcc 14, incompatible pointer types are now fatal by default. This patch fixes/workarounds the issue by casting affected pointers upon argument passing to function calls.