build: use quote to avoid error if variable is empty (#5053)

Problem identified by @irieger Ingmar Rieger, solves a problem that pops
up when Conan build provides the targets.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/heif.imageio/CMakeLists.txt

@@ -8,7 +8,7 @@ if (Libheif_FOUND)
     set (_static_libraries_found 0)
     foreach (_libeheif_library IN LISTS LIBHEIF_LIBRARIES)
         get_filename_component (_ext ${_libeheif_library} LAST_EXT)
-        list (FIND _static_suffixes ${_ext} _index)
+        list (FIND _static_suffixes "${_ext}" _index)
         if (${_index} GREATER -1)
             MATH (EXPR _static_libraries_found "${_static_libraries_found}+1")
         endif()