Do not error when libunwind-generic is not found, Not all
implementations of libunwind will provide this library therefore
do not make it hard error if its not found.

Upstream-Status: Submitted [https://github.com/libsdl-org/SDL/pull/5194]
Signed-off-by: Khem Raj <raj.khem@gmail.com>

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1358,7 +1358,7 @@ elseif(UNIX AND NOT APPLE AND NOT ANDROI
       if(HAVE_LIBUNWIND_H)
         # We've already found the header, so REQUIRE the lib to be present
         pkg_search_module(UNWIND REQUIRED libunwind)
-        pkg_search_module(UNWIND_GENERIC REQUIRED libunwind-generic)
+        pkg_search_module(UNWIND_GENERIC libunwind-generic)
         list(APPEND EXTRA_LIBS ${UNWIND_LIBRARIES} ${UNWIND_GENERIC_LIBRARIES})
       endif()
     endif()