cmake: prohibit LTO for heap_constants.c offsets source

Commit 518d842 ("lib: heap: compute Z_HEAP_MIN_SIZE from actual
struct layouts") added heap_constants.c to the offsets library, but
did not add the prohibit_lto compile option that offsets.c has.
When LTO is enabled, the compiler optimizes away the absolute symbols
emitted by GEN_ABSOLUTE_SYM, causing them to be missing from the
generated offsets.h.  This breaks builds that use Z_HEAP_MIN_SIZE
(via kernel/mempool.c) with LTO-enabled configurations.

Fix by adding heap_constants.c to the same set_source_files_properties
call that disables LTO for offsets.c.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

Author: Nicolas Pitre

CMakeLists.txt

@@ -1045,8 +1045,9 @@ target_include_directories(${OFFSETS_LIB} PRIVATE
   ${ARCH_DIR}/${ARCH}/include
   )
 
-# Make sure that LTO will never be enabled when compiling offsets.c
-set_source_files_properties(${OFFSETS_C_PATH} PROPERTIES COMPILE_OPTIONS $<TARGET_PROPERTY:compiler,prohibit_lto>)
+# Make sure that LTO will never be enabled when compiling offsets sources
+set_source_files_properties(${OFFSETS_C_PATH} ${ZEPHYR_BASE}/lib/heap/heap_constants.c
+  PROPERTIES COMPILE_OPTIONS $<TARGET_PROPERTY:compiler,prohibit_lto>)
 
 target_sources(${OFFSETS_LIB} PRIVATE ${ZEPHYR_BASE}/lib/heap/heap_constants.c)