Fix warning from autoconf
Fix warning from autoconf

autoconf reported this warning:

    configure.ac:263: warning: AC_CHECK_HEADERS("tensorflow/core/framework/graph.pb.h"): you should use literals

Signed-off-by: Stefan Weil <8d4c780fcfdc41841e5070f4c43da8958ba6aec0@weilnetz.de>

stweil/tesseract,tesseract-ocr/tesseract,tesseract-ocr/tesseract,tesseract-ocr/tesseract,tesseract-ocr/tesseract,stweil/tesseract,amitdo/tesseract,UB-Mannheim/tesseract,stweil/tesseract,amitdo/tesseract,amitdo/tesseract,stweil/tesseract,tesseract-ocr/tesseract,amitdo/tesseract,stweil/tesseract,amitdo/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract,UB-Mannheim/tesseract