fuzz: fix build failure on oss-fuzz

The build in oss-fuzz uses the latest git version of gnulib, whic produces the following error:

  make[2]: Entering directory '/src/libtasn1/fuzz'
    CC       corpus2array.o
  In file included from corpus2array.c:34:
  ../lib/gl/string.h:51:3: error: Please include config.h first."
   #error "Please include config.h first."

Fixes: #46 (closed)

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with other code
• This feature/change has adequate documentation added
• No obvious mistakes in the code