Allow .f90 files in kpp folder (wrf-model#1111)

TYPE: bug fix

KEYWORDS: .gitignore, kpp, f90

DESCRIPTION OF CHANGES:
Problem:
The *.f90 files are part of .gitignore. This is not entirely correct. There are instances where the
*.f90 files should be tracked by git.

Solution:
Exclude the *.f90 files under the kpp directory from this overly broad identification of files. There
are other directories where this fix could also be applied (such as for DA).

ISSUE:
Fixes wrf-model#1105 

LIST OF MODIFIED FILES:
M  .gitignore

TESTS CONDUCTED:
Compile and run WRF-Chem successfully

Author: zxdawn

.gitignore

@@ -1,3 +1,4 @@
+##############################################################################
 # This is the top-level .gitignore file for the WRF model                    #
 #                                                                            #
 # Filenames and wildcards added below will not be tracked by git in any      #

chem/KPP/kpp/kpp-2.1/.gitignore

@@ -0,0 +1,9 @@
+##############################################################################
+# These files are EXCLUDED from not being tracked, so they are still tracked #
+#                                                                            #
+# The leading "!" character is the NOT operator                              #
+#                                                                            #
+# The "**" syntax means "this directory level and recursively all            #
+# directories below this level"                                              #
+##############################################################################
+!**/*.f90