libfdtgen: set libfdt O2 flag when using libfdtgen

set libfdt O2 flag when using libfdtgen to speed up the generation
process

libfdtgen/CMakeLists.txt

diff --git a/libfdtgen/CMakeLists.txt b/libfdtgen/CMakeLists.txt
index 1859dee..c7455ba 100644
--- a/libfdtgen/CMakeLists.txt
+++ b/libfdtgen/CMakeLists.txt
@@ -14,6 +14,7 @@
 project(libfdtgen C)
 
 add_compile_options(-O2)
+target_compile_options(fdt PRIVATE -O2)
 add_library(fdtgen STATIC EXCLUDE_FROM_ALL fdtgen.c)
 target_include_directories(fdtgen PUBLIC include)
 target_link_libraries(fdtgen muslc utils fdt)