generic: fix PATH_MAX macro redefined in exec-cmd.c on macOS 15.4

generic: fix PATH_MAX macro redefined in exec-cmd.c on macOS 15.4

Fix an error while building target/linux x64 on macOS 15.4 host,
due to the PATH_MAX macro being redefined:

mkdir -p /Volumes/test/openwrt/build_dir/target-x86_64_musl/linux-x86_64/linux-6.6.86/tools/objtool && make O=/Volumes/test/openwrt/build_dir/target-x86_64_musl/linux-x86_64/linux-6.6.86 subdir=tools/objtool --no-print-directory -C objtool
exec-cmd.c:15:9: error: 'PATH_MAX' macro redefined [-Werror,-Wmacro-redefined]
   15 | #define PATH_MAX        4096
      |         ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/sys/syslimits.h:103:9: note: previous definition is here
  103 | #define PATH_MAX                 1024   /* max bytes in pathname */
      |         ^

exec-cmd.c is compiled as part of objtool to run on the host, and
therefore host headers are used, where PATH_MAX is already defined.
Using an old OpenWRT snapshot from 2025-02-16, where linux-6.6.77
used to build correctly, does not help. Reverting from Xcode 16.3 to
16.2 does not help either.

Signed-off-by: Georgi Valkov <gvalkov@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18530
Signed-off-by: Robert Marko <robimarko@gmail.com>