fix phi backend header should not include fluid header file, test=develop (#51244)

paddle/phi/CMakeLists.txt

@@ -55,7 +55,7 @@ file(
 )
 
 # generate inner headers include dir for users
-generate_unify_header(backends)
+generate_unify_header(backends EXCLUDES context_pool_utils.h)
 generate_unify_header(core EXCLUDES cuda_stream.h)
 generate_unify_header(infermeta)
 generate_unify_header(kernels SKIP_SUFFIX grad_kernel)