Make is_osx_mach_header declaration and definition consistent

clang 15 reported the type inconsistency.

Author: tautschnig

src/goto-programs/osx_fat_reader.cpp

@@ -85,7 +85,8 @@ osx_fat_readert::osx_fat_readert(
   if(!in)
     throw system_exceptiont("failed to read OSX fat header");
 
-  if(!is_osx_fat_header(reinterpret_cast<char *>(&(fh.magic))))
+  static_assert(sizeof(fh) >= 8, "fat_header is at least 8 bytes");
+  if(!is_osx_fat_header(reinterpret_cast<char *>(&fh)))
     throw deserialization_exceptiont("OSX fat header malformed");
 
   static_assert(

src/goto-programs/osx_fat_reader.h

@@ -74,6 +74,6 @@ class osx_mach_o_readert
   void process_sections_64(uint32_t nsects, bool need_swap);
 };
 
-bool is_osx_mach_object(char hdr[8]);
+bool is_osx_mach_object(char hdr[4]);
 
 #endif // CPROVER_GOTO_PROGRAMS_OSX_FAT_READER_H