Fix build failures with standard C 2023.

[emollier]

As initially identified in Debian bug #1097250, PGPLOT is affected by build failures caused by mismatching prototypes with Gcc 15:

In file included from PGPLOT.xs:16:
pgfun.c: In function ‘pgfun1’:
pgfun.c:78:7: warning: old-style function definition [-Wold-style-definition]
   78 | float pgfun1(x)
      |       ^~~~~~
pgfun.c:79:4: error: number of arguments doesn’t match prototype
   79 |    float *x; {
      |    ^~~~~
pgfun.c:29:7: error: prototype declaration
   29 | float pgfun1();
      |       ^~~~~~
pgfun.c: In function ‘pgfun2’:
pgfun.c:120:7: warning: old-style function definition [-Wold-style-definition]
  120 | float pgfun2(x)
      |       ^~~~~~
pgfun.c:121:4: error: number of arguments doesn’t match prototype
  121 |    float *x; {
      |    ^~~~~
pgfun.c:30:7: error: prototype declaration
   30 | float pgfun2();
      |       ^~~~~~
pgfun.c: In function ‘pgfunplot’:
pgfun.c:161:6: warning: old-style function definition [-Wold-style-definition]
  161 | void pgfunplot(visible,x,y,z)
      |      ^~~~~~~~~
pgfun.c:163:4: error: number of arguments doesn’t match prototype
  163 |    float *x,*y,*z; {
      |    ^~~~~
pgfun.c:31:8: error: prototype declaration
   31 | void   pgfunplot();
      |        ^~~~~~~~~

This patch adds missing function argument types in prototypes, and casts a couple of indirect function calls, to ensure compatibility with the standard C 2023, now selected by default by gcc 15 and later.

[emollier]

I had a peek at the failing integration tests on Ubuntu, but I believe the failure is unrelated:

  /usr/bin/ld: cannot find -lX11: No such file or directory

I'm under the impression that the severe coverage regression is caused by some of the failing Ubuntu tests, although honestly I'm not too sure what went wrong precisely.

Anyways, in hope this helps,
Have a nice day, :)
Étienne.

[mohawk2]

Thank you! As soon as the CI finishes a second time (PDL's CI removed X11, so I had to add X11 for this module's CI), I'll merge.