Remove #include <fp.h> and other obsolete code from pngpriv.h.

Fixes:

./../compat/libpng/arm/../pngpriv.h:524:16: fatal error: 'fp.h' file not found
  524 | #      include <fp.h>
      |                ^~~~~~

https://github.com/pnggroup/libpng/commit/893b8113f04d408cc6177c6de19c9889a48faa24
--- compat/libpng/pngpriv.h.orig	2026-03-31 22:50:44.000000000 -0500
+++ compat/libpng/pngpriv.h	2026-03-31 22:53:26.000000000 -0500
@@ -514,18 +514,7 @@
     */
 #  include <float.h>
 
-#  if (defined(__MWERKS__) && defined(macintosh)) || defined(applec) || \
-    defined(THINK_C) || defined(__SC__) || defined(TARGET_OS_MAC)
-   /* We need to check that <math.h> hasn't already been included earlier
-    * as it seems it doesn't agree with <fp.h>, yet we should really use
-    * <fp.h> if possible.
-    */
-#    if !defined(__MATH_H__) && !defined(__MATH_H) && !defined(__cmath__)
-#      include <fp.h>
-#    endif
-#  else
 #    include <math.h>
-#  endif
 #  if defined(_AMIGA) && defined(__SASC) && defined(_M68881)
    /* Amiga SAS/C: We must include builtin FPU functions when compiling using
     * MATH=68881