Fix compilation with gcc10 wrt multiple definitions issue

gcc10 now defaults to -fno-common (ref: https://gcc.gnu.org/gcc-10/changes.html ). Now with compiled with gcc10, multiple definitions are found which causes linker error as:

*  --with-tcl:
```
/usr/bin/ld: table/table3D/cfunc.o:/builddir/build/BUILD/ngspice-32/ngspice/tclspice/src/xspice/icm/../../../src/include/../xspice/icm/dlmain.h:8: multiple definition of `coreitf'; table/dlmain.o:/builddir/build/BUILD/ngspice-32/ngspice/tclspice/src/xspice/icm/dlmain.c:45: first defined here
```

*  --enable-oldapps
```
/bin/ld: frontend/com_history.o:(.data+0x4): multiple definition of `cp_maxhistlength'; ngsconvert.o:(.bss+0xa4): first defined here
collect2: error: ld returned 1 exit status
```

This commit fixes these issues.

src/ngsconvert.c

@@ -24,7 +24,6 @@ FILE *cp_err = NULL;
 FILE *cp_curin = NULL;
 FILE *cp_curout = NULL;
 FILE *cp_curerr = NULL;
-int cp_maxhistlength;
 bool cp_debug = FALSE;
 char cp_chars[128];
 bool cp_nocc = TRUE;

src/xspice/icm/dlmain.h

@@ -5,6 +5,6 @@
 #include "ngspice/mifproto.h"
 #include "ngspice/dllitf.h"
 
-struct coreInfo_t *coreitf;
+extern struct coreInfo_t *coreitf;
 
 #endif /* dlmain.h */