rviz_ogre_vendor: fix macOS SDK detection in vendored OGRE

[AKooshan82]

Building rviz_ogre_vendor on macOS fails on Apple Silicon. With Ninja I get:

fatal error: 'Availability.h' file not found

and with Unix Makefiles I get a corrupted flags.make:

OgreMain/CMakeFiles/OgreMain.dir/flags.make:10: *** missing separator.  Stop.

Both come from the same block in the pinned OGRE v1.12.10 CMakeLists.txt:

set(XCODE_ATTRIBUTE_SDKROOT macosx)
if(CMAKE_GENERATOR STREQUAL "Unix Makefiles")
  execute_process(COMMAND xcodebuild -version -sdk "${XCODE_ATTRIBUTE_SDKROOT}" Path | head -n 1 OUTPUT_VARIABLE CMAKE_OSX_SYSROOT)
  string(REGEX REPLACE "(\r?\n)+$" "" CMAKE_OSX_SYSROOT "${CMAKE_OSX_SYSROOT}")
else()
  set(CMAKE_OSX_SYSROOT macosx)
endif()

execute_process() doesn't run a shell, so | head -n 1 isn't a pipe. Those
four tokens are passed to xcodebuild as arguments, and current Xcode rejects
-n:

xcodebuild: error: option '-n' is no longer supported

xcodebuild then prints its usage text, that lands in CMAKE_OSX_SYSROOT, and
the newlines end up embedded in flags.make. Older Xcode accepted -n as
dry-run and ignored the rest, which is presumably why this went unnoticed.

The else() branch is the Ninja path. Setting CMAKE_OSX_SYSROOT to the literal
string "macosx" only works with the Xcode generator; everything else gets
-isysroot macosx, no SDK, and no Availability.h.

This patch replaces the block with a single xcrun call, which returns a clean
absolute path.

Upstream OGRE has already removed this block entirely (it's gone as of 14.6.0),
so this is only needed while rviz_ogre_vendor pins v1.12.10.

Tested on macOS 26.5.2 (arm64), Xcode 26.6.
rviz_ogre_vendor builds cleanly with this applied.

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