Fixing typo in IDL AACGM DLM

[egthomas]

This pull request fixes a small typo in the IDL DLM implementation of AACGM_v2_GetDateTime that causes the following compilation warning:

aacgmdlm.c: In function ‘IDLAACGM_v2_GetDateTime’:
aacgmdlm.c:240:14: warning: ISO C forbids zero-size array ‘outargv’ [-Wpedantic]
   240 |     IDL_VPTR outargv[0];
       |              ^~~~~~~
 aacgmdlm.c:272:5: warning: array subscript 0 is outside array bounds of ‘IDL_VARIABLE *[0]’ {aka ‘IDL_VARIABLE *[]’}     [-Warray-bounds]
   272 |     IDL_StoreScalar(outargv[0],IDL_TYP_LONG,(IDL_ALLTYPES *) &yr);
       |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 aacgmdlm.c:240:14: note: while referencing ‘outargv’
   240 |     IDL_VPTR outargv[0];
       |              ^~~~~~~

On this branch, the code compiles cleanly without any warnings.

[ecbland]

Reproduced the bug on develop, and the fix on this branch works as described. Merging now.