"unexpected OpenMP directive" for included header file

#ifndef CALLOMP_H
 #define CALLOMP_H

void callOmp(){
 #ifdef _OPENMP
 #pragma omp parallel for
 #endif
 for(int idx=0;idx<100;++idx){
 }
 }

#endif // CALLOMP_H

#include <vector>
 #include "callomp.h"

int main()

{ return 0; }

QT -= gui

CONFIG += c++17 console
 CONFIG -= app_bundle

QMAKE_LFLAGS += -fopenmp
 QMAKE_CXXFLAGS += -fopenmp

DEFINES += QT_DEPRECATED_WARNINGS

SOURCES += \
 main.cpp

HEADERS += \
 callomp.h