nrfconnect: fix warning when compiling c files (#15565)

diff --git a/config/nrfconnect/app/enable-gnu-std.cmake b/config/nrfconnect/app/enable-gnu-std.cmake
index 34c5f56..5f31e1a 100644
--- a/config/nrfconnect/app/enable-gnu-std.cmake
+++ b/config/nrfconnect/app/enable-gnu-std.cmake
@@ -1,3 +1,6 @@
 add_library(gnu17 INTERFACE)
-target_compile_options(gnu17 INTERFACE -std=gnu++17 -D_SYS__PTHREADTYPES_H_)
+target_compile_options(gnu17
+    INTERFACE 
+        $<$<COMPILE_LANGUAGE:CXX>:-std=gnu++17>
+        -D_SYS__PTHREADTYPES_H_)
 target_link_libraries(app PRIVATE gnu17)