Add json_fwd.hpp to the Bazel singleheader-json target

The generator for BUILD.bazel only listed single_include/nlohmann/json.hpp
in the singleheader-json cc_library's hdrs. Bazel's sandboxing only exposes
declared headers, so a consumer of :singleheader-json that includes
<nlohmann/json_fwd.hpp> failed with "file not found" (verified with Bazel
9.2.0). Meson and include.zip already ship json_fwd.hpp; the Bazel target
was missing it.

Add single_include/nlohmann/json_fwd.hpp to the hdrs list and regenerate
BUILD.bazel. Verified that `bazel build //:json //:singleheader-json`
succeeds and that a consumer cc_binary depending on :singleheader-json can
now include <nlohmann/json_fwd.hpp>.

Overlaps #5621, which adds json_view.hpp to the same hdrs list in
gen_bazel_build_file.cmake; that PR will need a rebase.

#5716 item 2

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

BUILD.bazel

@@ -77,6 +77,7 @@ cc_library(
     name = "singleheader-json",
     hdrs = [
         "single_include/nlohmann/json.hpp",
+        "single_include/nlohmann/json_fwd.hpp",
     ],
     includes = ["single_include"],
     visibility = ["//visibility:public"],

cmake/scripts/gen_bazel_build_file.cmake

@@ -48,6 +48,7 @@ cc_library(
     name = "singleheader-json",
     hdrs = [
         "single_include/nlohmann/json.hpp",
+        "single_include/nlohmann/json_fwd.hpp",
     ],
     includes = ["single_include"],
     visibility = ["//visibility:public"],