Fix warning "cc1plus: warning: ‘-Werror=’ argument...

Fix warning "cc1plus: warning: ‘-Werror=’ argument ‘-Werror=implicit-function-declaration’ is not valid for C++"

src/CMakeLists.txt

@@ -153,7 +153,7 @@ set_property(TARGET kdenliveLib PROPERTY CXX_STANDARD 14)
 SET(KDENLIVE_CXX_FLAGS  "${DEFAULT_CXX_FLAGS} -Wall -pedantic -Wextra")
 SET(KDENLIVE_CXX_FLAGS "${KDENLIVE_CXX_FLAGS} -Wcast-qual -Wcast-align -Wfloat-equal -Wpointer-arith")
 SET(KDENLIVE_CXX_FLAGS "${KDENLIVE_CXX_FLAGS} -Wunreachable-code -Wchar-subscripts -Wcomment -Wformat")
-SET(KDENLIVE_CXX_FLAGS "${KDENLIVE_CXX_FLAGS} -Werror-implicit-function-declaration -Wmain -Wmissing-braces")
+SET(KDENLIVE_CXX_FLAGS "${KDENLIVE_CXX_FLAGS} -Wmain -Wmissing-braces")
 SET(KDENLIVE_CXX_FLAGS "${KDENLIVE_CXX_FLAGS} -Wparentheses -Wsequence-point -Wreturn-type -Wswitch")
 SET(KDENLIVE_CXX_FLAGS "${KDENLIVE_CXX_FLAGS} -Wuninitialized -Wreorder -Wundef -Wshadow -Wwrite-strings")
 SET(KDENLIVE_CXX_FLAGS "${KDENLIVE_CXX_FLAGS} -Wmissing-noreturn -Wsign-compare -Wsign-conversion -Wunused")