packfile: drop a repeated enum declaration

When compiling under Apple LLVM version 9.1.0 (clang-902.0.39.2) with
"make DEVELOPER=1 DEVOPTS=pedantic", the compiler says

    error: redeclaration of already-defined enum 'object_type' is a GNU
    extension [-Werror,-Wgnu-redeclared-enum]

According to https://en.cppreference.com/w/c/language/declarations
(section "Redeclaration"), a repeated declaration after the definition
is only legal for structs and unions, but not for enums.

Drop the belated declaration of enum object_type. It seems that each
includer of packfile.h includes the definition of the enum before
including packfile.h.

Signed-off-by: Beat Bolli <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bbolli

packfile.h

@@ -6,7 +6,6 @@
 /* in object-store.h */
 struct packed_git;
 struct object_info;
-enum object_type;
 
 /*
  * Generate the filename to be used for a pack file with checksum "sha1" and