[gradsusr] Fix compilation with -Werror=implicit-function-declaration

[Orion Poplawski]

Fedora has started building all C code with 
-Werror=implicit-function-declaration - see 
https://fedoraproject.org/wiki/Changes/Fedora26CFlags for more info.

grads fails to build with:

gxwmap.c: In function 'gxrobi':
gxwmap.c:593:9: error: implicit declaration of function 'dequal' 
[-Werror=implicit-function-declaration]
      if (dequal(lonmn,0.0,1e-7) || dequal(lonmx,360.0,1e-7)) return (1);
          ^~~~~~

gxeps.c: In function 'initepsf':
gxeps.c:997:8: error: implicit declaration of function 'gahistory' 
[-Werror=implicit-function-declaration]
         gahistory("his",strncat(buf,fout,255), NULL);
         ^~~~~~~~~

gxcntr.c: In function 'gxshplin':
gxcntr.c:869:4: error: implicit declaration of function 'gaprnt' 
[-Werror=implicit-function-declaration]
     gaprnt(0,pout);
     ^~~~~~

The attached patch fixes.


-- 
Orion Poplawski
Technical Manager                     303-415-9701 x222
NWRA/CoRA Division                    FAX: 303-415-9702
3380 Mitchell Lane                  orion at cora.nwra.com
Boulder, CO 80301              http://www.cora.nwra.com
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