Fix malformed patch file

The patch file had incorrect unified diff formatting. Fixed by:
- Properly showing the removal of the unconditional 'use m_thermochem' line
- Properly showing the addition of the conditional #:if chemistry block
- Ensuring correct line prefixes (-, +, and space for context)

Author: sbryngelson

packaging/spack/fix-chemistry-disabled.patch

@@ -2,20 +2,17 @@ diff --git a/src/common/m_derived_types.fpp b/src/common/m_derived_types.fpp
 index 1234567..7654321 100644
 --- a/src/common/m_derived_types.fpp
 +++ b/src/common/m_derived_types.fpp
-@@ -11,10 +11,16 @@ module m_derived_types
+@@ -11,7 +11,12 @@ module m_derived_types
      use m_constants  !< Constants
 
      use m_precision_select
+-    use m_thermochem, only: num_species
 +#:if chemistry
-     use m_thermochem, only: num_species
++    use m_thermochem, only: num_species
 +#:else
 +    ! Chemistry disabled: use dummy value for num_species
 +    integer, parameter :: num_species = 1
 +#:endif
 
      implicit none
 
-     !> Derived type adding the field position (fp) as an attribute
-     type field_position
-         real(wp), allocatable, dimension(:, :, :) :: fp !< Field position
-